Which is a prime number?
17
A prime number has exactly two factors: 1 and the number itself.
12 = 2 × 6 = 3 × 4, so it has factors other than 1 and 12.
15 = 3 × 5, and 20 = 2 × 10 = 4 × 5, so both are composite.
17 is not divisible by 2, 3 or any number up to √17 (about 4.1), so its only factors are 1 and 17.
Hence, the prime number is 17.
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