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GMAT Quant: Thoughts on Remainders

For a start, give these problems a try.  A complete explanation will come at the end of the discussion.

 

1) When positive integer N is divided by positive integer J, the remainder is 14.  If N/J = 134.08, what is value of J?

  1. 22
  2. 56
  3. 78
  4. 112
  5. 175

2) When positive integer N is divided by positive integer P, the quotient is 18, with a remainder of 7.  When N is divided by (P + 2), the quotient is 15 and the remainder is 1.  What is the value of N?

  1. 151
  2. 331
  3. 511
  4. 691
  5. 871

3) P and Q are both positive integers.  When P is divided by Q, the remainder is some positive integer D, and when P is divided by (Q + 3), the remainder is also D.  If P/Q = 1020.75 and P/(Q + 3) = 816.6, then which of the following gives the correct set of {D, Q}?

  1. {6, 12}
  2. {6, 15}
  3. {9, 12}
  4. {9, 15}
  5. {15, 24}

I will discuss those questions at the end of this article.

 

Division Terminology

Let’s look at division carefully and think about the parts.  Suppose we divide 33 by 4.  Of course, 4 goes into 33 eight times, with a remainder of 1.  Let’s talk about the official names of this cast of characters.

33 = the dividend = the number being divided

4 = the divisor = the number doing the dividing; the number by which you divide

8 = the integer quotient = the integer that results from a whole number of divisions

1 = the remainder

We have to add a caveat here.  Notice: here we are talking about positive integers only —- living in the magical fairyland where the only numbers that exist are positive integers, where skies are not cloudy all day.  Unless you live on a farm where the barnyard animals all sing in unison, you don’t get to stay here forever.

Of course, numbers in the real world aren’t like that, and if you prance through the GMAT Quantitative Section as if it’s a magical fairyland where all the numbers are positive integers, this section will utterly decimate you.  In the real world that involves all possible numbers, this process looks a bit different.  For example, if you type 33/4 into your calculator, your calculator will tell you

33/4 = 8.25

That decimal, 8.25, is the quotient, the actual real-world mathematical quotient.   In this article, for clarity, I will refer to this one as the “decimal quotient.”

Notice, first of all — the integer part of the decimal quotient is exactly equal to the integer quotient.  It has to be.  In fact, we can go a little further:  Let’s look at this process both with words and with numbers:

{dividend/divisor} = decimal quotient = integer quotient + {remainder/divisor}

{33/4}= 8.25 = 8 + {1/4}

We divide the dividend (33) by the divisor (4), and we get the decimal quotient.  The integer part of the decimal question is the first piece of the last sum, the integer quotient.  What’s crucially important is — the decimal part of the decimal quotient equals the final fraction:

decimal part of decimal quotient ={remainder/divisor}

Virtually any problem on the GMAT that gives you a decimal quotient is relying on this particular formula.  It is crucial for answering #1 and #3 above.

 

Rebuilding the Dividend

Let’s go back to the integer relationships:

{dividend/divisor} =  integer quotient + {remainder/divisor}

If you are given the divisor, the integer quotient, and the remainder, then you can rebuild the dividend.  In particular, notice that “divisor” is the denominator of both fractions, so we if multiple all three terms by “divisor”, it cancels in two of the three terms:

dividend = (integer quotient)*(divisor) + remainder

That formula is pure gold in questions which give you an integer quotient, a divisor, and a remainder.  Even if one or two of those three are in variable form, it allows us to set up an algebraic relationship we can solve.  This is crucial for answering #2 above.

 

Practice Questions

It may be, at this point, you want to give those three questions another attempt before reading the solutions.  Here’s yet another practice question:

4) http://gmat.magoosh.com/questions/305

 

Practice Question Solutions

1) We know that

decimal part of decimal quotient ={remainder/divisor}

So 0.08, the decimal part of the decimal quotient, must equal the remainder, 14, divided by the divisor J.

0.08 = 14/J

0.08*J = 14

J = 14/0.08 = 1400/8 = 700/4 = 350/2 = 175

So J = 175, answer = E.

2) Use the rule dividend = (integer quotient)*(divisor) + remainder to translate each sentence.

The first sentence becomes N = 18P + 7.  The second equation becomes N = (P + 2)*15 + 1, which simplifies to N = 15P + 31.  These are ordinary simultaneous equations (http://magoosh.com/gmat/2012/gmat-quant-how-to-solve-two-equations-with-two-variables/).  Since they both equal N already, let’s set them equal and solve for P.

18P + 7 = 15P + 31

3P + 7 = 31

3P = 24

P = 8

 Now that we know P = 8, we can just plug in.  The product 15*8 is particularly easy to do, without a calculator, by using the “doubling and halving” trick.  Double 15 to get 30, and take half of 8 to get 4 —- 15*8 = 30*4 = 120.  So

N = 15(8) + 31 = 120 + 31 = 151

N = 151, answer = A.

3) Here, we have to use

decimal part of decimal quotient ={remainder/divisor}

to translate each act of division.  The first one tells us 0.75 = D/Q, and the second one tells us that 0.60 = D/(Q + 3).  These are also two simultaneous equations.  Multiply both to get rid of the fractions.

D = 0.75*Q

D = 0.60*(Q + 3) = 0.60*Q + 1.80

Both are equal to D, so set them equal to each other and solve for Q.

0.75*Q = 0.60*Q + 1.80

0.15*Q = 1.80

Q = 1.80/0.15 = 180/15 = 60/5 = 12

Now that we know Q = 12, we can find D = 0.75*Q = (3/4)*12 = 9.  So D = 9 and Q = 12.  Answer = C.

 

About the Author

Mike McGarry is a Content Developer for Magoosh with over 20 years of teaching experience and a BS in Physics and an MA in Religion, both from Harvard. He enjoys hitting foosballs into orbit, and despite having no obvious cranial deficiency, he insists on rooting for the NY Mets. Follow him on Google+!

28 Responses to GMAT Quant: Thoughts on Remainders

  1. KL September 11, 2014 at 2:05 am #

    Hi, if I fully understand simultaneous equations, would it be sufficient to solve these type of questions?

    • Mike
      Mike September 11, 2014 at 9:53 am #

      Dear Kailee,
      If you know the ideas on this page (decimal – remainder relationship, rebuilding the dividend, etc.), then the GMAT will throw nothing harder at you than can be handled by straightforward simultaneous equations. Does this make sense?
      Mike :-)

  2. motoki September 5, 2014 at 9:47 am #

    Dear Mike

    I have a question about (1).
    If the decimal part of the decimal quotient is remainder/divisor, then why is 0.08 (2/25) not R(remainder)/D ?
    Divisor J must be equal 25.
    but answer is J =175.

    • Mike
      Mike September 5, 2014 at 10:27 am #

      Dear Motoki,
      I’m happy to respond. :-) This is always a tricky thing about ratios — if we know the ratio of A/B, we don’t necessarily know the individual values A & B. For example, if we know that the ratio of males to females at a company is 3:2, that doesn’t mean that just five people work at that company! :-)
      Much in the same way, in this problem, it absolutely must be true that remainder/divisor is in the ratio of 0.08 = 2/25, but it would be a HUGE mistake to deduce from this fact that the remainder must be 2 and the divisor must be 25. In fact, we are told that the remainder is 14. All we know from that decimal part of the division is a ratio, not individual quantities.
      This mistake about ratios is a very easy mistake to make, a very tempting trap in a variety of GMAT situations involving ratios. Always be very discerning about ratios — about exactly what you do know and exactly what you don’t know. When you understand how to think about ratios and execute proportional thinking on the GMAT, that’s an enormously powerful problem-solving tool.
      Does all this make sense?
      Mike :-)

      • Motoki September 7, 2014 at 9:03 pm #

        Thank you Mike.
        Finally I could comprehend the ratio!!!

        • Mike
          Mike September 8, 2014 at 9:56 am #

          Dear Motoki,
          You are quite welcome. :-) Best of luck to you!
          Mike :-)

  3. aisha May 15, 2014 at 12:26 am #

    Thanks Mike- you’re phenomenal as always. This is a great lesson

    But I’m intrigued– why have we not seen this rule before

    Meaning, decimal part = remainder/divisor… i haven’t seen it anywhere

    • Mike
      Mike May 15, 2014 at 12:26 pm #

      Dear Aisha,
      Thank you for your kind words. I’m very glad you have found this helpful. Yes, it’s funny: not many people talk about this math factoid. It doesn’t often show up on the GMAT, and when it does, it is usually on the harder math questions. It’s peculiar that no one else mentions it, because it could be important on a question.
      Mike :-)

  4. Niels February 23, 2014 at 1:03 pm #

    Dear Mike,

    Thanks for all your great methods and suggestions. It was of great help to me and it made me nail my Gmat. Thanks!

    • Mike
      Mike February 23, 2014 at 3:15 pm #

      Niels,
      Congratulations! You are more than welcome! Best of luck in your future!
      Mike :-)

  5. Nadeen Harding October 30, 2013 at 5:51 pm #

    I am confused about a part of the explanation given for problem three. If the decimal part of the decimal quotient is remainder/divisor, then why is 0.75 D/Q and not R(remainder)/D ?

    • Mike
      Mike October 30, 2013 at 6:33 pm #

      Nadeen,
      That problem is a little confusing because it uses different letters. Notice, in that problem D is the remainder and Q is the divisor. Don’t be fooled by letters alone — you always have to look at the concepts behind the letters.
      Mike :-)

  6. Sadie Griffiths July 26, 2013 at 5:43 pm #

    very nice, very helpful

    • Mike
      Mike July 26, 2013 at 7:03 pm #

      Sadie,
      Thank you for the kind words. Best of luck to you.
      Mike :-)

  7. MensaMember July 20, 2013 at 3:05 am #

    Dear Mike, Thanks for yet another terrific blog post!

    BTW, how does this shortcut to solve Q # 3 look? :-)

    Since, it is given P/Q = 1020.75 and remainder is D, D/Q= 0.75 which is 3:4. The only answer choice that satisfies this condition is ‘C’

    Thanks!

    • Mike
      Mike July 20, 2013 at 3:13 pm #

      Dear Mensa Member,
      Yes, that’s a great shortcut. Of course, if this had been a true GMAT-style question, probably there would have been more than one choice for which that ratio was true. I didn’t think to include that when I wrote the question, but that’s precisely the kind of thing they would see.
      Does this make sense?
      Mike :-)

      • MensaMember July 21, 2013 at 12:28 am #

        Yes, that would be a deadly GMAT trap, if one was not careful. Thanks, Mike, for the insight!

        • Mike
          Mike July 21, 2013 at 1:44 pm #

          You are more than welcome.
          Best of luck to you.
          Mike :-)

          • Katie August 31, 2014 at 4:06 am #

            That’s what I did as well, and I would have chosen “C” on the GMAT, but I doubted because it seems to completely disregard the second remainder/divisor which is 60/100 or 3/5. How does that come into play?

            • Mike
              Mike August 31, 2014 at 10:36 am #

              Dear Katie,
              I’m happy to respond. :-) Remember, in that second division, the divisor is (Q + 3). We would have to add three to the second number in each answer pair, and then check the ratios — for {9/12}, the division we would check is 9/(12 + 3) = 9/15 = 3/5.
              Does this make sense?
              Mike :-)

  8. sarika July 19, 2013 at 3:06 pm #

    Hi Mike,
    I did not understand the below part. In words, it says divisor/dividend but in numbers dividend/divisor which is actually correct. Could you please elaborate more on this as in connection between word and number formula below?

    {divisor/dividend} = decimal quotient = interger quotient + {divisor/dividend}

    {33/4}= 8.25 = 8 + {1/4}

    • Mike
      Mike July 19, 2013 at 3:29 pm #

      Dear Sarika,
      I’m sorry. There were a couple typos in that line, I believe I have corrected everything. Thank you for pointing this out, and let me know if there is anything that still doesn’t make sense.
      Mike :-)

  9. jeff March 7, 2013 at 11:07 pm #

    Thank you. I studied four semesters of math while in college (25= years ago) and never learned this. Frightening. But wow will this help with problems like this on the test.

    • Mike
      Mike March 8, 2013 at 10:26 am #

      Jeff — it’s funny — I think most of us heard about this when we were in late grade school, but at that point, mathematical niceties were lost on us, and then going through puberty blurred everything. Unfortunately, many math teacher do not return to these basic subjects when students are older and able to understand the underlying logic. For example, it helps to think about this topic with algebraic symbolism, but when we first learned it in the fourth grade, we hadn’t learned algebra yet! I’m glad you found this helpful. Thank you for your kind words.
      Mike :-)

  10. Alex September 25, 2012 at 4:00 pm #

    Thanks! Thanks to you I finally got a good method for remainders problem.

    • Mike
      Mike September 26, 2012 at 11:23 am #

      You are quite welcome.
      Mike :-)

  11. GMAT Prep July 9, 2012 at 1:28 pm #

    Very nice set of material here. Thanks for the information.

    • Mike
      Mike July 9, 2012 at 6:59 pm #

      Thank you for your kind words. Best of luck to you.
      Mike :-)


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