Why is 87 not a prime number?

Sum of the digits of 87 = 8+7 =15(a multiple of 3) which means that 87 is divisible by 3.

Therefore, 87 is not a Prime number as it has more than two factors which are 1, 87, 3 etc.

In order to prove that a given number is prime we shall show that the given number has more than two factors. There is no need to find all the factors.