4th ed Number Properties CH10 Problem Set Pg133 q10

[tushaw]

Question 10:
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Is the sum of integers a and b divisible by 7?

(1) a is not divisible by 7.
(2) a-b is divisible by 7.
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I understand how both choices are "insufficient" by themselves. But I do not get how both (1) and (2) are sufficient together. Below is my reasoning. Please correct if I'm wrong:

- From (1) we know 'a' is not a multiple of 7.
- (2) indicates that 'a-b' is div by 7, so 'b' has to be a non-multiple of 7 too.

- When two non-multiples of 7 are added together, the result is either a multiple of 7 or non-multiple of 7.
- Therefore we cannot conclude 'a+b' is divisible by 7.

Answer - E

The problem set mentions 'C' as the answer

[aboinpally]

tushar -> I think you did not post the actual question !

[tushaw]

The post now has the question listed

[jnelson0612]

Please see this thread which discusses this question: tricky-question-t10312.html