Which number is prime?

Which number is prime?

• A. 15
• B. 18
• C. 21
• D. 19

Answer: (D)

A prime number is a natural number greater than 1 that has only two positive divisors: 1 and itself. To check small numbers, you only need to test divisibility by the smallest primes up to the square root of the number. For 19, the square root is about 4.36, so you test divisibility by 2 and 3. 19 is not divisible by 2 or 3, so it has no divisors besides 1 and 19, meaning it is prime. The other numbers have divisors other than 1 and themselves (15 can be divided by 3 and 5; 18 by 2 and 3; 21 by 3 and 7), so they are not prime.

A prime number is a natural number greater than 1 that has only two positive divisors: 1 and itself. To check small numbers, you only need to test divisibility by the smallest primes up to the square root of the number.

For 19, the square root is about 4.36, so you test divisibility by 2 and 3. 19 is not divisible by 2 or 3, so it has no divisors besides 1 and 19, meaning it is prime. The other numbers have divisors other than 1 and themselves (15 can be divided by 3 and 5; 18 by 2 and 3; 21 by 3 and 7), so they are not prime.