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Divisiblity problem:

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Source: — Data Sufficiency |
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by das.ashmita » Wed Oct 31, 2012 12:33 am
Hi Manpreet

The Ans is C
(1) a is not divisible by 7.
for a=15 and b=1 a+b=16 not divisible by 7
for a=15 and b=-1 a+b=14 divisible by 7

not suff
(2) a - b is divisible by 7.
for a=21 and b=7 a-b is divisible by 7 and a+b=14 divisible by 7
for a=15 and b=1 a-b is divisible by 7 but a+b=16 is not divisible by 7

not suff

combining the (1) and (2)

for a=15 and b=1 a-b is divisible by 7 but a+b=16 is not divisible by 7
for a=20 and b=-1 a-b is divisible by 7 but a+b=16 is not divisible by 7

Suff

Hope it helps
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by \'manpreet singh » Wed Oct 31, 2012 8:16 pm
Thanks Ashmita ,nice explanation!
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by theachiever » Thu Nov 01, 2012 6:02 pm
1.a is not divisible by 7

This data alone is insufficient since value of b is not known

2.a-b is divisible by 7.

This statement alone is not sufficient

Combining both statements assume values of a and b accordingly.We will get a clear answer without any ambiguity.

Hence Answer is C


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