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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and equations ensures a solution.

If x, y and z are integers, is x odd?

(1) yz = x

(2) x - y = z In the original condition we have 3 variables (x,y,z) and we need 3 equations to match the number of variables and equations. Since there is 1 each in 1) and 2), E is likely the answer. Using both 1) & 2) together, x=y+z=yz thus y=z=even --> x=even and the answer is no. However, if y=even, z=odd -->then x=odd, but out of scope(since yz=even) and also y=z=odd --> x=even, is also out of scope (since yz=odd). therefore, the only answer is y=z=even leading us to x=even. The answer is no, therefore the conditions sufficient. The answer is C.
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Re: If x, y and z are integers, is x odd? [#permalink]

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11 Sep 2015, 02:45

Bunuel wrote:

If x, y and z are integers, is x odd?

(1) yz = x

(2) x - y = z

Kudos for a correct solution.

(1) yz = x. Insufficient. x can be even or odd depending on the values of y and z (2) x - y = z. x=y+z.Insufficient. x can be even or odd depending on the values of y and z

Using both, only one case exists, y,z=2,2 and x=4 Answer C

This is a good example of a problem that does not seem to give us very much information in the stem or in the statements that is related to the question. However, by doing some algebra we can try to find a relationship.

First, let’s assess Statement 1. Statement 1 tells us nothing about whether x is even or odd, as we do not know anything about if y and z are odd. Therefore, statement 1 is not sufficient (and you can eliminate answer choices 1 & 4, or A & D).

Next, let’s assess Statement 2 (and remember—we are first assessing each statement on its own). Statement 2 runs into a similar problem as statement 1. We can isolate x here, to make the equation x = z + y, but without knowing anything about z and y, statement 2 is also insufficient (so you can eliminate answer choice 2 or B).

When considering the statements together, many test-takers will simply say that without knowing anything about y and z still, we cannot answer the question. However, we can do a little algebra here to gain some information. Since we know from statement 1 that x = yz and from statement 2 that x = y + z, we can say:

yz = y + z

yz – z = y

z(y-1) = y

Because y is an integer, we know that either y is even or y-1 is even. However, because an even times anything is an even, if y-1 is even then z(y-1) must equal an even number and y and y-1 cannot both be even. Therefore, the only way z(y-1) can be even is if z is even and y-1 is odd. Since we know that z is even and that y is even, we know that zy is also even. So, x must be even, because x = zy. The answer to our question, then, is ‘always no’ and the statements are sufficient together (answer choice 3 or C in Data Sufficiency).
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