How to solve this one quickly?

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r_walid: Senior | Next Rank: 100 Posts; Posts: 43; Joined: Wed Jan 21, 2009 2:24 pm; Thanked: 1 times

by r_walid » Thu Nov 05, 2009 10:04 am

which of the following has a terminating decimal?

10/189

5/196

16/225

25/144

39/128

The answer is easy when you have enough time. I'm posting this not to ask for the answer but to ask what or how can i solve this in 2 minutes without a calculator? there has to be a short way for doing so and I need your help on how. so please explain with detail not only the process but the way to solve it quickly, what tricks are there to solve this in 2 minutes only?

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Source: Beat The GMAT — Problem Solving | Thu Nov 05, 2009 10:04 am

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by beatthegmat » Thu Nov 05, 2009 10:06 am

Moved this post to Problem Solving forum.

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NikolayZ: Master | Next Rank: 500 Posts; Posts: 124; Joined: Thu Jun 18, 2009 5:33 am; Thanked: 35 times

by NikolayZ » Thu Nov 05, 2009 10:22 am

backsolving backsolving backsolving.
Notice - backsolve from E. Works for you more frequently then backsolving from A

You need to prime factorize numerator and denominator.
If denominator will consist of 2's or 5's - it is 100% terminating decimal.

E)
39/128=(13*3)/(13*2*2*2*2)==>(3/2*2*2*2) => terminating decimal.

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mp2437: Master | Next Rank: 500 Posts; Posts: 177; Joined: Thu Aug 14, 2008 11:59 am; Thanked: 25 times

by mp2437 » Thu Nov 05, 2009 11:01 am

how is 128 divisible by 13?

I understand and agree with the statement that if denominators prime factors are 2 and 5 only, then it will be terminating, however, still not seeing how you reduced the fraction.

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NikolayZ: Master | Next Rank: 500 Posts; Posts: 124; Joined: Thu Jun 18, 2009 5:33 am; Thanked: 35 times

by NikolayZ » Thu Nov 05, 2009 11:18 am

oopsie

too many silly mistakes for the day, preceding the g-day. sorry all. I am out

however! this one is still the right choice

it ll be
(13*3)/(2^6) still GOTCHA ! hehe .terminating !

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palvarez: Master | Next Rank: 500 Posts; Posts: 199; Joined: Sat Oct 24, 2009 4:43 pm; Thanked: 22 times; GMAT Score:710

by palvarez » Fri Nov 13, 2009 12:25 am

(a) First convert fractions to lowest terms. (b) Then look for any denominator whose factors contain only 2 and 5.

GMAT trap: giving fractions in not-lowest terms, thereby trapping folks who know only (b).

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