SAT Math Multiple Choice Practice Question 23: Answer and Explanation

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Question: 23

1. A number, n, is defined as a "tweener" if both n - 1 and n + 1 are prime. Which of the following numbers is a tweener?

A. 2
B. 8
C. 30
D. 36
E. 48

Correct Answer: C

Explanation:

C. Don't fall into the trap of thinking that 1 is prime. Two numbers, 1 and 0, are called -"special." Thus, 2 isn't a tweener because 2 - 1 = 1 and 1 isn't prime. The number 8 isn't a tweener, because 8 + 1 = 9 and 9 isn't prime, either. (It's 3 × 3.) But 30 is a tweener because 30 - 1 = 29, which is prime, and 30 + 1 = 31, which is also prime. After you've found the answer, you can move on to the next question; there's no need to check the last two options, which are wrong.

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