How many of the numbers between 20 and 40 are prime numbers?

How many of the numbers between 20 and 40 are prime numbers?

A) 3
B) 4
C) 5
D) 6
E) 7

Recall that a prime number is a number that only has itself and 1 as a divisor.

• Remove all the evens since the are divisible by 2 (20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40)
• Remove all the remaining numbers divisible by 3 (21, 24, 27, 30, 33, 36, 39)
• Remove all the remaining numbers divisible by 5 (25, 35)

We're left with (23, 29, 31, 37)

This is 4 numbers which is answer B