has exactly 2 factors.
how do we know that 71 is a prime number? if 71 were not a prime number, then it would be divisible by at least one prime number less than or equal to √71 ≈ 8.4. since 71 cannot be divided evenly by 2, 3, 5, or 7, we know that 71 is a prime number.
71 is never a clue in the find the factors puzzles. 