Which of the following is NOT a prime number?

SOLUTION

Prime number: A natural number larger than unity is a prime number if it does not have other divisors except for itself and unity.

As properties of prime number (p):

p > 3, p² - 1 is completely divisible by 24.

From options:

1) 61

⇒ (61² - 1)/24 = (3721 - 1)/24 = 155

2) 71

⇒ (71² - 1)/24 = (5041 - 1)/24 = 210

3) 69

⇒ (69² - 1)/24 = (4761 - 1)/24 = 198.33

4) 67

⇒ (67² - 1)/24 = (4489 - 1)/24 = 187

∴ From above calculation 69 is not a prime number as it does not satisfy prime number properties.