Question

How many two-digit prime numbers have a units digit of $7$?

Answer 1

Five

Explanation:

There are 9 two-digit numbers with a units digit of 7:

17, 27, 37, 47, 57, 67, 77, 87, 97

Of these, 4 are composite numbers:

27 = 3³

57 = 3×19

77 = 7×11

87 = 3×29

The remaining 5 are prime numbers:

17, 37, 47, 67, 97