GMAT Work Rate Problems


Runner taking off representing calculating gmat rate problems - image by magosh

GMAT rate problems might seem intimidating, but they’re not so bad once you get the fundamental concepts down. We’ll show you how to master this common type of GMAT word problem in this post, then give you some practice problems with answers and explanations!

GMAT Rate and Work Rate Problems: Main Concepts

Big Idea #1: The “ART” Equation

You may be familiar with the distance equation, D = RT (“distance equals rate times time”), sometimes remembered as the “dirt” equation. It turns out, that equation is just a specific instance of a much more general equation. In that equation, R, the rate, is distance per time, but in non-distance problems, rate can be anything over time — wrenches produced per hour, houses painted per day, books written per decade, etc. In these cases, typical of work problems, we are no longer concerned with “distance” per time, but with the amount of something produced per time. We use A to represent this amount (the number of wrenches, the number of houses, etc.), and the equation becomes A = RT. Sometimes folks remember this as the “art” equation.

Here’s a simple mnemonic. When you travel, you are moving on the Earth, which is made of dirt, so for traveling & distance you use D = RT. Work problems involve machines, and machines make things —– making is creation, and creation is the essence of art, so use the A = RT equation. (I know, I know, what comes out of most machines is hardly worthy of aesthetic elevation, but it works for a mnemonic!)

Big Idea #2: Rates are Ratios

The word “rate” and the word “ratio” have the same Latin root: in fact, they also share a Latin root with the “rationality” of our minds, but that’s a discussion that would bring up to our noses into Pythagorean and Platonic philosophies. The point is: a rate is a ratio, that is to say, a fraction. Technically, any fraction, any ratio, in which the numerator and the denominator have different units is a rate. Fuel efficiency (mpg) and price per unit and most baseball fractions (ERA, BA, OBP, SLG, etc.) are rates. Currency rates and exchanges rates are common financial market rates that, ironically, almost never appear on the GMAT —- go figure! Most GMAT rates have time in the denominator, and it’s a rate of how fast work is being done or how fast something is being produced or accomplished.

The fact that rates are ratios means: we can solve these problems by setting up proportions and using proportional thinking! As you will see in the solutions below, that’s an extremely powerful strategy for solution.

Big Idea #3: Add Rates

The vast majority of work problems on the GMAT involve two people or two machines and comparisons of their individual production to their combined production. The questions will often give you information about times and about amounts, and what you need to know is: you can’t add or subtract times to complete a job and you can’t add or subtract amounts of work; instead, you add and subtract rates.

(rate of A alone) + (rate of B alone) = (combined rate of A & B)

Here A and B can be two people, two machines, etc. The extension of this idea is that if you have N identical machines, and each one works at a rate of R, then the combined rate is N*R.

Big Idea #4: Understand Speed and Average Speed

Rate is another word for speed. One common source of errors with GMAT rate problems is that all three variables have to be in the same units. If you travel at 30 mph for 10 minutes, you do not go 30*10 = 300 miles!

Here’s an example:

\(\frac{30 \text{miles}}{\text{hour}} = \frac{30 \text{miles}}{60 \text{minutes}} = \frac{x \text{miles}}{10 \text{minutes}}\) \(\frac{1 \text{mile}}{2 \text{minutes}} = \frac{x \text{miles}}{10 \text{minutes}}\)

Cross-multiply, and you get 10 = 2x. So, x must equal 5 miles.

Many trickier rate questions ask about “average speed” or “average velocity” (for GMAT purposes, those two are identical). The formula for average speed is:

\( \text{Average speed}=\frac {\text{Total distance}}{\text{Total time}}\)
 
For a single trip at one speed, there’s nothing particularly mysterious about this question. This concept becomes much trickier in two-leg trips, especially trips in which the car travels at one speed in one leg, and at another speed in another leg. You can never simply average the two velocities given, and that will always be a tempting incorrect choice on the GMAT. You always need to apply D = RT separately in each leg of the trip, and then you need to add results from the individual legs to find the total distance and the total time.

With just these four ideas, you can unlock any GMAT work rate problem. At this point, you may want to go back and give another attempt at those three practice questions. Follow carefully how they are applied in the solutions below.

Practice GMAT Rate Problems

For practice, here are some GMAT rate problems for you! The last two are challenging.

1) A car drives 300 miles at 30 mph, and then 300 miles at 60 miles per hour. What is the car’s average speed, in mph?





Show answer and explanation

In order to figure out the average velocity, we need to know both the total distance and the total time. From the question, we know the total distance is 600 miles. We need to figure out the time of each leg separately. In the first leg, T = D/R = 300/30 = 10 hr. In the second leg, T = D/R = 300/60 = 5 hours. The total time is 10 + 5 = 15 hours. The average velocity, total distance divided by total time, is 600/15 = 40 mph. Answer = B.

2) A car drives for 3 hours at 40 mph and then drives 300 miles at 60 mph. What is the car’s average speed, in mph?





Show answer and explanation

In the first leg, we know time and rate, so find distance: D = RT = (3)*(40) = 120 miles. In the second leg, we know distance and rate, so find time: T = R/D = 300/60 = 5 hours. Total distance = 120 + 300 = 420 miles. Total time = 3 + 5 = 8. Average velocity = 420/8 = 210/4 = 105/2 = 52.5 mph. Answer = D.

3) For the first 150 miles of a trip, a car drives at v mph. For the next 200 miles, the car drives at (v + 25) mph. The average speed of the whole trip is 35 mph. Find the value of v.





Show answer and explanation

The distance of the first leg is 150 miles, and the rate is v, so the time of the first leg is:

\(\text{t}_1 = \frac{150}{\text{v}}\)
 
The distance for the second leg is 200, and the rate is v+25, so the time of the second leg is:

\(\text{t}_2 = \frac{200}{\text{v}+25}\)
 
The total distance was 350 miles, and the average speed was 35 mph, so the total time of the trip must have been T = D/R = 350/35 = 10 hours. At this point, the algebra becomes hairy, so I will just plug in numbers from the answer choices.

Choice A. If v = 20 mph, then v + 25 = 45 mph. The first leg takes 150/20 = 7.5 hours, and the last leg 200/45 takes way more than three hours, so this total time is well over 10 hours. This choice is not correct.

Choice B. If v = 25, then v + 25 = 50. The first leg takes 150/25 = 6 hours. The second leg takes 200/50 = 4 hours. The total is 10 hours, which is the correct value, so this is the correct answer choice. Answer = B.

4) A car travels at one speed for 4 hours, and then at twice that speed for 6 hours. The average velocity for the whole 10-hour trip is 40 mph. Find the initial speed in mph.





Show answer and explanation

If the average velocity for the 10 hour trip is 40 mph, that means the total distance is D = RT = (40)*(10) = 400 miles. The distance in the first leg is d1 = RT = 4v. The distance in the second leg is d2 = RT = (2v)*(6) = 12v. The total distance is the sum, 4v + 12v = 16 v. Set this equal to the numerical value of the total distance.

400 = 16v → 100 = 4v → 25 = v

So the initial speed is v = 25 mph. Answer = A.

5) Running at the same rate, 8 identical machines can produce 560 paperclips a minute. At this rate, how many paperclips could 20 machines produce in 6 minutes?





Show answer and explanation

“Running at the same rate, 8 identical machines can produce 560 paperclips a minute.” That 560 is a combined rate of 8 machines —- 560 = 8*R, so the rate of one machine is R = 560/8 = 70 paperclips per minute.

“At this rate, how many paperclips could 20 machines produce in 6 minutes?” Well, the combined rate of 20 machines would be Rtotal = 20*70 = 1400 pc/min. Now, plug that into the “art” equation: A = RT = (1400)*(6) = 8400 pc. Answer = C.

6) Jane can make a handcrafted drum in 4 weeks. Zane can make a similar handcrafted drum in 6 weeks. If they both work together, how many weeks will it take for them to produce 15 handcrafted drums?





Show answer and explanation

Method I: the rates solution

“Jane can make a handcrafted drum in 4 weeks. Zane can make a similar handcrafted drum in 6 weeks.” Jane’s rate is (1 drum)/(4 weeks) = 1/4. Zane’s rate is (1 drum)/(6 weeks) = 1/6. The combined rate of Jane + Zane is

R = 1/4 + 1/6 = 3/12 + 2/12 = 5/12

That’s the combined rate. We need to make 15 drums — we have a rate and we have an amount, so use the “art” equation to solve for time:

T = A/R = 15/(5/12) = 15*(12/5) = (15/5)*12 = 3*12 = 36

BTW, notice in the penultimate step, the universal fraction strategy: cancel before you multiply (Tip #3: https://magoosh.com/gmat/2012/can-i-use-a-calculator-on-the-gmat/. Jane and Zane need 36 weeks to make 15 drums. Answer = B.

Method II: the proportion solution

“Jane can make a handcrafted drum in 4 weeks. Zane can make a similar handcrafted drum in 6 weeks.” Let’s find the LCM of 4 and 6 — that’s 12 weeks. In a 12 week period, Jane, making a drum every 4 weeks, makes three drums. In a 12 week period, Zane, making a drum every 6 weeks, makes two drums. Therefore, in a 12 weeks period, they produce 5 drums between the two of them. If they make 5 drums in 12 weeks, they need triple that time, 36 weeks, to make 15 drums. Therefore, Jane and Zane need 36 weeks to make 15 drums. Answer = B.

7) Machines P and Q are two different machines that cover jars in a factory. When Machine P works alone, it covers 1500 jars in m hours. When Machines P and Q work simultaneously at their respective rates, they cover 1500 jars in n hours. In terms of m and n, how many hours does it take Machine Q, working alone at its constant rate, to cover 1500 jars?





Show answer and explanation

This is a particularly challenging, one because we have variables in the answer choices. I will show an algebraic solution, although a numerical solution (https://magoosh.com/gmat/math/word-problems/variables-in-gmat-answer-choices-algebraic-approach-vs-numerical-approach/) is always possible.

“Machines P and Q are two different machines that cover jars in a factory. When Machine P works alone, it covers 1500 jars in m hours. When Machines P and Q work simultaneously at their respective rates, they cover 1500 jars in n hours. In terms of m and n, how many hours does it take Machine Q, working alone at its constant rate, to cover 1500 jars? ”

Since the number “1500 jars” appears over and over, let’s arbitrarily say 1500 jars = 1 lot, and we’ll use units of lots per hour to simplify our calculations.

P’s individual rate is (1 lot)/(m hours) = 1/m. The combined rate of P & Q is (1 lot)/(n hours) = 1/n. We know

(P’s rate alone) + (Q’s rate alone) = (P and Q’s combined rate)

(Q’s rate alone) = (P and Q’s combined rate) – (P’s rate alone)

(Q’s rate alone) = 1/n – 1/m = m/ (nm) – n/ (nm) = (m – n)/(nm)

We now know Q’s rate, and we want the amount of 1 lot, so we use the “art” equation.

1 = [(m – n)/ (nm)]*T

T = (mn)/(m – n)

Answer = D

8) Working together, 7 identical pumps can empty a pool in 6 hours. How many hours will it take 4 pumps to empty the same pool?





Show answer and explanation

Correct Answer: E
Click here for the full video and text explanation!

That’s all there is to GMAT work rate problems! Do you still have questions? Leave a comment below!

Ready to get an awesome GMAT score? Start here.

Most Popular Resources

Author

  • Mike MᶜGarry

    Mike served as a GMAT Expert at Magoosh, helping create hundreds of lesson videos and practice questions to help guide GMAT students to success. He was also featured as "member of the month" for over two years at GMAT Club. Mike holds an A.B. in Physics (graduating magna cum laude) and an M.T.S. in Religions of the World, both from Harvard. Beyond standardized testing, Mike has over 20 years of both private and public high school teaching experience specializing in math and physics. In his free time, Mike likes smashing foosballs into orbit, and despite having no obvious cranial deficiency, he insists on rooting for the NY Mets. Learn more about the GMAT through Mike's Youtube video explanations and resources like What is a Good GMAT Score? and the GMAT Diagnostic Test.

12 Responses to GMAT Work Rate Problems

  1. Melisa S September 11, 2016 at 1:44 pm #

    I’m getting lost at this line… can anyone help me understand how this logically makes sense:

    (Q’s rate alone) = 1/n – 1/m = m/ (nm) – n/ (nm) = (m – n)/(nm)

    Thanks!

    • Magoosh Test Prep Expert
      Magoosh Test Prep Expert September 11, 2016 at 5:42 pm #

      Hi Melisa,

      So, let’s first briefly walk through what we know. We know that “(P’s rate alone) + (Q’s rate alone) = (P and Q’s combined rate).” We are also told that “(P’s rate alone) = (1/m)”, while “(P and Q’s combined rate) = (1/n).” Now, if we plug this information in and solve for “(Q’s rate alone)”, we get the following equation:

      (Q’s rate alone) = (P and Q’s combined rate) – (P’s rate alone)
      (Q’s rate alone) = 1/n – 1/m

      Next, if we want to add these two fractions together, both fractions require the same denominator. So, we’ll have to multiply “m” to numerator and denominator of “1/n”. We will also multiply “n” to the numerator and denominator of “1/m”.

      (Q’s rate alone) = m/(nm) – n/(nm)

      Remember, we aren’t changing these two individual fractions (i.e., (1/n) and (1/m)). We are just rephrasing them. For example, “m/(nm) = 1/n” if we cancel out the “m” in the numerator and denominator. Okay, now that the two fractions have the same denominator, we can subtract them together.

      (Q’s rate alone) = (m-n)/(nm)

      Hope this helps!

  2. Shivangi June 23, 2016 at 3:01 am #

    Your theories and concepts always help me alot , be it any topic!!
    Thanks Mike :’)

  3. Rishi December 13, 2015 at 11:21 am #

    Hi Mike!

    I had a quick question about that one video where you mention you CANNOT cross simplify with proportions. For a specific question on the video: “A machine can produce 36 staplers in 28 minutes, how many staplers can it produce in 1 hour and 45 minutes?”

    Here is what I did, and I arrived at the correct answer. I set the equation up as you did initially, but instead of using a proportion, I cross multiplied between 9/7 and 105/1. Since we are making 9 staplers every 7 minutes, how many staplers would we make in 145 minutes. Thus I just multiplied using these two numbers and in fact I did simplify 7 and 105 to get 15. From there 15(9) = 135. The next thing you mentioned is not to cross multiply so I got concerned whether this approach was feasible or not. Did I just get lucky or is setting up a proportion totally different?

  4. Ravi August 8, 2014 at 1:00 am #

    Hi Mike

    Are these enough for all WRT problems that appear on GMAT?

    Btw, your approach proved to be quite helpful! 🙂

    Thanks

    • Mike MᶜGarry
      Mike August 8, 2014 at 10:38 am #

      Dear Ravi,
      I’m glad you found this helpful. 🙂 I’m going to give you a piece of advice. Don’t ask the question “Are these enough?” or “Is this enough?” or “Can I consider myself done after this?” Those are the questions of mediocrity, and they consistently lead to limited performances. Consider the questions “What else can I understand about this?” and “How can I understand this topic more deeply?” and “What else can I do to improve myself?” Those are the questions of excellence. When you follow those questions and live by them, you are able to bring your best to any challenge.
      Does all this make sense?
      Mike 🙂

  5. Sagnik Baksi January 2, 2014 at 4:12 pm #

    Dear Mike,

    I know you cover frequency of GMAT topics on this website, but in terms of the OG 13 how many 700+ level questions does it have for the Quant and DS respectively?
    Also I see people posting Quant Scores of 51 how is this possible if there is only 37 Quant Questions?

    • Mike MᶜGarry
      Mike January 2, 2014 at 5:11 pm #

      Dear Sagnik,
      First of all, please understand that the entire idea of a “700+” question is very vague, not at all well defined. Roughly, it means questions that are among the hardest one might see on the GMAT. To know the exact level of any particular question, we would have to know the percentage of folks who get the question correct. If fewer than 10% got the question correct, then maybe it could be called a “700+” question. I don’t think many questions in the OG fit this description — maybe 10% or 15% of the hardest ones.
      Also, I think you need to understand a little more about how the GMAT is scored. See these two posts:
      https://magoosh.com/gmat/2012/what-is-the-gmat-cat-computer-adaptive-test/
      https://magoosh.com/gmat/2013/gmat-score-percentiles/
      The score of 51 on Quant is a scaled score that represents a percentile rank. What matters is not simply how many questions one gets right, but the difficulty level of each question. The computer does a very complicated calculation to get from one’s individual correct & incorrect questions to that scaled score.
      Does all this make sense?
      Mike 🙂

  6. maria January 6, 2013 at 9:51 am #

    Thank you!! I´ve been struggling with this type of questions and with your explanation I could finally solve them! I do the Gmat in one week!

    • Mike MᶜGarry
      Mike January 6, 2013 at 6:06 pm #

      Maria:
      Thank you for your kind words. Best of luck to you.
      Mike 🙂

  7. Fernanda D. December 29, 2012 at 9:21 pm #

    Thanks for the post. That was extremely helpful! Undoubtedly it will get me through these type of questions much faster now. Best,

    • Mike MᶜGarry
      Mike December 31, 2012 at 9:21 am #

      Fernanda,
      I am very glad you found this helpful. Best of luck to you!
      Mike 🙂


Magoosh blog comment policy: To create the best experience for our readers, we will only approve comments that are relevant to the article, general enough to be helpful to other students, concise, and well-written! 😄 Due to the high volume of comments across all of our blogs, we cannot promise that all comments will receive responses from our instructors.

We highly encourage students to help each other out and respond to other students' comments if you can!

If you are a Premium Magoosh student and would like more personalized service from our instructors, you can use the Help tab on the Magoosh dashboard. Thanks!

Leave a Reply