SSAT Math Question 397: Answer and Explanation
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Question: 397
8. If two numbers, P and Q, have an average of 80, and P – Q ≠ 0, which of the following must be true?
- A. P + Q = 80
- B. P – Q = 40
- C. P =30 and Q = 50
- D. 80 – P = Q – 80
Correct Answer: D
Explanation:
D
Start by writing the average equation: 
= 80. Then multiply both sides by 2: P + Q = 160. Check the answers. None of choices (A), (B), and (C) match the equation you wrote. Manipulate choice (D) to see if it matches, by adding P and 80 to both sides. It does.
Plug in numbers for P and Q, following the restrictions given, such as P = 60 and Q = 100. Only choice (D) is valid when you plug P and Q into the answer choices.