Gmatprep 2 question

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bkobilov: Junior | Next Rank: 30 Posts; Posts: 22; Joined: Thu Jul 15, 2010 9:49 pm; Thanked: 1 times

by bkobilov » Thu Sep 30, 2010 8:21 am

Please help.

If x and y are positive integers, is xy a multiple of 8 ?

1. The greatest common divisor of x and y is 10
2. The least common divisor multiple of x and y is 100

OA: C

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Source: Beat The GMAT — Data Sufficiency | Thu Sep 30, 2010 8:21 am

shovan85: Community Manager; Posts: 991; Joined: Thu Sep 23, 2010 6:19 amFollowed by: 24 members; Location: Bangalore, India; Thanked: 146 times

by shovan85 » Thu Sep 30, 2010 8:31 am

IMO C

from 1: GCF of x and y is 10. That means x and y both are multiple of 10. Hence, we can write
x=10k
y=10k' where k and k' are +ve integers.
But we cannot say from this info if xy is multiple of 8

from2: LCM of x and y is 100. Same we cannot say whether xy is multiple of 8.

Now combine both
from 1 xy = 10k*10k' = 100(k*k')
and
from 2 we know the LCM of xy is 100.

Thus the only possibility when LCM(x,y) = 100 and GCF(x,y) = 10 is when x=10 and y=10.
So 10*10 = 100 is not a multiple of 8.

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