John took a test that had 60 questions...

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John took a test that had 60 questions... [#permalink]

15 Dec 2012, 09:48

John took a test that had 60 questions numbered from 1 to 60. How many of the questions did he answer correctly?

This is a DS question from GMAT OG12 (problem 29) but I was hoping to get some assistance with a specific part of solving the question not the actual solution itself.

My question is how do you determine the number of odd numbers as well as the number of even numbers in a data set? I am aware that there are 30 odd and 30 even in this data set (60 questions from 1 to 60) but what if I had a data set from like 34-101 questions (how would i solve for the number of odd numbers and even numbers)? I seem to remember a number property formula to easily do this but cant recall what it was. Thanks for your help!

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Re: John took a test that had 60 questions... [#permalink]

15 Dec 2012, 10:38

jmuduke08 wrote:

For 34 - 101 use the formula below

(101 - 34 + 1)/2 = 34

34 even numbers and 34 odd numbers between 34 and 101 inclusive.

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Re: John took a test that had 60 questions... [#permalink]

15 Dec 2012, 10:56

I believe I was thinking of an interval formula to determine how many numbers at a certain interval are within a data set. for example, how many intervals of 7 are there in the range 280-3000? thanks again!

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Re: John took a test that had 60 questions... [#permalink]

09 Jan 2013, 20:39

jmuduke08 wrote:

Divide the count (2721) by 7

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Re: John took a test that had 60 questions... [#permalink]

11 Jan 2013, 04:45

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jmuduke08 wrote:

First find the first multiple of 7 in the range. 280 is divisible by 7

7*40 = 280

Then find the last multiple in the range. When you divide 3000 by 7, you are left with a remainder of 4. So the last multiple must be 2996.
7*428 = 2996

From 40 to 428, you have 428 - 40 + 1 = 389 multiples of 7 in the given range.

Note: The concept of (3000 - 280)/7 may not always give you the right answer.
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Re: John took a test that had 60 questions... [#permalink] 11 Jan 2013, 04:45

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John took a test that had 60 questions...

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