What is the probability of selecting a prime number out of the following set?

{2, 6, 9, 17, 21, 47, 63, 71, 81}

How many prime numbers are between 40 and 60?

How many integers between 2 and 61 are prime numbers, inclusive?

Which of the following sets of numbers contain all prime numbers?

How many prime numbers between 20 and 40 are divisible by 9?