For positive integers a and b, a/b. Which of the following c

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alanforde800Maximus: Master | Next Rank: 500 Posts; Posts: 187; Joined: Tue Sep 13, 2016 12:46 am

by alanforde800Maximus » Tue Oct 04, 2016 4:54 am

For positive integers a and b, a/b = 0.6. Which of the following cannot be the value of a?

a) 42
b) 63
c) 105
d) 137
e) 174

Please assist with above problem.

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Source: Beat The GMAT — Problem Solving | Tue Oct 04, 2016 4:54 am

regor60: Master | Next Rank: 500 Posts; Posts: 416; Joined: Thu Oct 15, 2009 11:52 am; Thanked: 27 times

by regor60 » Tue Oct 04, 2016 6:07 am

alanforde800Maximus wrote:For positive integers a and b, a/b = 0.6. Which of the following cannot be the value of a?

a) 42
b) 63
c) 105
d) 137
e) 174

Please assist with above problem.

0.6 reduces to 3/5, so any "a" is a multiple of 3. Test the values above to see which are. 137 looks prime to me, so test the other 3. Answer is D

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by [email protected] » Tue Oct 04, 2016 9:47 am

Hi alanforde800Maximus,

regor60's approach to this question is really efficient, but there are a variety of different ways to 'brute force' this question (and taking one of those approaches would be preferable to just 'staring' at the prompt and hoping that an efficient solution comes to you).

Here, we're told that A and B are POSITIVE INTEGERS, which is an interesting 'restriction' since we're also told that A/B = .6

A/B = .6
A = (.6)(B)

So we need a pair of integers that will 'balance out' this equation. At this point, you could TEST THE ANSWERS - plug each option in for "A" and solve for B. Whichever answer does NOT lead to an integer value for B is the answer.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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