A Guide to the GRE/Odds and Evens

Adding & Subtracting
An odd plus an odd equals an even.

5 + 7 = 12 = even

An even plus an even equals an even.

6 + 4 = 10 = even

An odd plus an even equals an odd.

6 + 5 = 11 = odd

Multiplying & Dividing
An odd multiplied by an odd equals an odd.

5(5) = 25 = odd

An odd multiplied by an even equals an even.

5(2) = 10 = even

An even multiplied by an even equals an even.

4(8) = 32 = even

'''When in doubt, test this with two known numbers. If unsure, try 1 + 3 or (2)(5) and see what type of number it yields.'''

Practice
1. xy is even and xz is odd. Is y odd or even?

2. a + b + c is even. If a is odd, then could b and c be

both odd?

both even?

odd and even?

3. (f + g)h is an odd number. If f is even, is g + h odd or even?

Answers to Practice Questions
1. y must be even.

The product of two numbers is only odd if both numbers are odd. Since xz is odd, both x and z must be odd. However, xy is even. This means that between x and y, one must be even. Since x is odd, y must be even.

2. b and c must be odd and even, not necessarily in that order.

An odd plus an odd equals an even, as does an even plus an even. Only an odd plus an even equals an odd. Since a + b + c is even and a is odd, b + c must be odd. This means that, between b and c, one must be odd, and one must be even. If they were both odd or even, b + c would be even, meaning a + b + c would be even, but it isn't.

3. g + h must be even.

Only an odd and another odd multiply to make an odd. Therefore, since (f + g)h is odd, f + g must be odd. Since f is even, g must be odd, since only an even and an odd add up to an odd. h also must be odd, since it multiplies to make an odd. Therefore, since an odd plus an odd equals an even, g + h must be even.