Bunuel
If x, y, and z are consecutive integers in increasing order, which of the following must be true?
I. xy is even.
II. x – z is even.
III. xz is even.
(A) I only
(B) II only
(C) III only
(D) I and II only
(E) I and III only
Since x, y, and z are consecutive integers, those integers consists of either 2 even and 1 odd or 2 odd and 1 even number. Let’s now analyze our Roman numerals:
I. xy is even.
Since x and y are consecutive integers, one will be even and one will be odd. The product even x odd = even holds for all integers. Roman numeral I must be true.
II. x – z is even.
Since x and z are either both odd or both even, and since odd - odd = even and even - even = even, the difference of x and z will always be even. Roman numeral II is true.
III. xz is even.
Recall that x and z are either both odd or both even. If x and z are both even, the product is even.
However, if x and z are both odd, the product is odd. Thus, xz is not necessarily even. Roman numeral III is not true.
Answer: D