A prime number less than 100 is selected randomly. What is the probability that its square has units digit 7?
These are the squares of ALL prime numbers below 100:
(4, 9, 25, 49, 121, 169, 289, 361, 529, 841, 961, 1369, 1681, 1849, 2209, 2809, 3481, 3721, 4489, 5041, 5329, 6241, 6889, 7921, 9409)
As you can see, there isn't a single one with its units digit = 7. Therefore, the probability is zero!.
