Data Sufficiency

M7MBA wrote:Is M < 0?

1) -M = |-M|
2) M^2 = 9.

The OA is C.

I am a little confused here. Shouldn't it be A the correct answer? Experts, can you help me here?

1) -M = |-M|

Case 1: If M = 0, then -M = |-M| => -0 = |-0| => 0 = 0. The answer is No.
Case 2: If M < 0, then -1*-|M| = |-1*-|M|| => |M| = |M|. The answer is Yes.
Case 3: If M > 0, then -M = M. Not possible. This is not a valid case.

Thus, M can be 0 or less than M. No unique answer. Insufficient.

2) M^2 = 9.

=> |M| = 3 => M = 3 or M = -3. No unique answer. Insufficient.

(1) and (2) together

From both the statements, we get M = -3 => M < 0. Sufficient.

The correct answer: C

Hope this helps!

-Jay
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