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GMATPrep - Another trap

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givemeanid: Master | Next Rank: 500 Posts; Posts: 277; Joined: Sun Jun 17, 2007 2:51 pm; Location: New York, NY; Thanked: 6 times; Followed by:1 members

GMATPrep - Another trap

by givemeanid » Mon Sep 03, 2007 9:09 am

I came across this while taking a GMATPrep test.

If k is a positive integer, then 20k is divisible by how many different positive integers?

1. k is prime
2. k = 7

So It Goes

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agps: Master | Next Rank: 500 Posts; Posts: 101; Joined: Tue Aug 07, 2007 1:45 am; Followed by:1 members

by agps » Mon Sep 03, 2007 9:22 am

I would answer D (either)

2) (easier) 20k = 20*7 = you know the number then you can determine all possible factors, sufficient.

1) any prime is divisible only by itself and 1. any prime multiplied by 20 will be divisible by it's factors and the factors of 20 (2, 4, 5, 10) so it's sufficient.

OA?

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givemeanid: Master | Next Rank: 500 Posts; Posts: 277; Joined: Sun Jun 17, 2007 2:51 pm; Location: New York, NY; Thanked: 6 times; Followed by:1 members

by givemeanid » Mon Sep 03, 2007 9:37 am

That's why its a trap son

Try it again. D is not the OA

So It Goes

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agps: Master | Next Rank: 500 Posts; Posts: 101; Joined: Tue Aug 07, 2007 1:45 am; Followed by:1 members

by agps » Mon Sep 03, 2007 9:48 am

hmmm,
is it because of 2? 2 is a prime, but also a factor of 20, then, not knowing what prime nuber it is you can't be sure if you have 4 different factors (for 2) or 5 (for any other prime number)

it that case, the answer would be B.

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givemeanid: Master | Next Rank: 500 Posts; Posts: 277; Joined: Sun Jun 17, 2007 2:51 pm; Location: New York, NY; Thanked: 6 times; Followed by:1 members

by givemeanid » Mon Sep 03, 2007 12:47 pm

Now you got it. You have to consider 2 (or 5) as a value of k!

So It Goes

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