Which of the following numbers is both prime and exactly one less than a perfect square?
- A.3 ✓
- B.8
- C.15
- D.24
Explanation: 3 = 4 − 1 = 2² − 1, and 3 is prime. 8 = 9−1 isn't prime, 15 = 16−1 isn't prime, and 24 = 25−1 isn't prime.
Class 6 IMO Mathematics Olympiad practice · Number Sense · Factors, Multiples & Primes
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