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sum & divisibility

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Source: — Data Sufficiency |
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by syr » Wed Jun 03, 2009 5:14 pm
1) a is not divisible by 7

Cannot determine. Since we dont know b.

2) a - b is divisible by 7

In this case, taking examples --
18 - 4 divisible by 7, but 18 + 4 not divisible by 7
10 - 3 divisible by 7, but 10 + 3 not divisible by 7
7 - 0 divisible by 7, but 7 + 0 divisible by 7

3) Taking both stmt together -
we can drop out 7 - 0, since 7 is divisible by 7.
Thus we can conclude that a + b is not divisible by 7

Hence C

I chose C

OA please.
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by pathaniaus » Thu Jun 04, 2009 2:25 am
it is indeed C. But I am still not understanding. Could u explain in more detail please??
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by sreak1089 » Mon Jun 08, 2009 6:45 am
This is how, I would solve it:

1) a is not div by 7
2) a-b is divisible by 7

From statement 1: a is not div by 7 (Note: Alone is not sufficient)

From statement 2: a-b = 7k (Note: Alone is not sufficient)

=> b = 7k + a (from statement 2 & statement 1 (a not div by 7))
=> b is not div by 7

a+b = (a-b) + 2b = 7k + 2b => a+b not div by 7 (since 2b not div. by 7)


Ans: C
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