Learn to master this common type of GMAT word problem!
Rate is another word for speed, and the equation for this is D = RT (which some people remember as the “dirt” equation). D = distance, R = rate (a.k.a. speed, a.k.a. velocity), and T = time. One common source of errors 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!
It’s also helpful to remember, as etymology would suggest, that a “rate” is a “ratio”, which in term is a fraction. With rates, we can always use ratios and always set up proportions. One way to find how far one goes at 30 mph in 10 minutes is to say
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:
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.
For practice, here are some average velocity questions in Two-Leg trips. 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?
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?
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.
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.
Practice Question Explanations
1) 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) 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) The distance of the first leg is 150 miles, and the rate is v, so the time of the first leg is
The distance for the second leg is 200, and the rate is v, so the time of the second leg is
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) 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.
So the initial speed is v = 25 mph. Answer = A.