Which of the following number sets is different from the other three?

Key insight: the odd set contains a composite number while the other sets contain only prime numbers.

• Check {7, 31, 21, 37}: 21 = 3 × 7, so 21 is composite. Therefore this set includes a non-prime.

• Check {5, 13, 17, 29}: all four numbers are prime.

• Check {3, 7, 11, 23}: all four numbers are prime.

• Check {11, 31, 13, 43}: all four numbers are prime.

Conclusion: the set {7, 31, 21, 37} is different because it contains the composite number 21, while each of the other sets contains only prime numbers.