SAT Math Multiple Choice Practice Question 49: Answer and Explanation
5. Given that o is odd and that e is even, which of the following must be odd?
B. (o - e)(o + e)
C. o(o + 1)
D. (e + 1)(o - 1)
E. e(o - e)
Correct Answer: B
B. All the answer choices involve multiplication, and the only way to get an odd number as the answer to a multiplication problem is to multiply two odd numbers. Thus, you can throw out Choices (A) and (E) right away because you multiply by e in both of them. Choices (C) and (D) are also no good: If o is odd, o - 1 and o + 1 are both even. These calculations leave you with Choice (B). As always, check to make sure that your answer really does work. An odd plus or minus an even is always odd, so both o - e and o + e are odd, and the product must also be odd.