ISEE Word Problems Question 248: Answer and Explanation

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Question: 248

12. For all integers x and y, let x y be defined as x y = (x + y)² – x – y. For example, 2 3 = 5² – 2 – 3 = 20. Which one of the following is an odd number?

A. 3 4
B. 3 5
C. 4 6
D. none of the above

Correct Answer: D

Explanation:

The answer is D

A "weird symbol" defined to equal an equation is simply a different form of function notation, so plug in numbers as directed by the defined equation. Test each answer. For choice (A), 3 4 = 7² - 3 - 4 = 42. Eliminate choice (A). For choice (B), 3 5 = 8² - 3 - 5 = 56. Eliminate choice (B). For choice (C), 4 6 = 10² - 4 - 6 = 90. Eliminate choice (C), leaving choice (D).

You can avoid testing choice (A) because you already know that an odd number and an even number yield an even number.

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