How do you determine when a three digit number is prime number?

Answer 1

To determine if any number is prime, one method is to check if it can be divided by any of the prime numbers less than its square root. The square root of 100 is 10 and the square root of 1000 is approximately 31.6, so any 3 digit number (100 to 999) needs to be tested for divisibility only with the prime numbers 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, and 31. Another test is that any number ending in 0, 2, 4, 5, 6, 7, or 8 cannot be prime.