Is 642 a prime number

Answer 1

Just by looking at it, without trying any factors to see if they work, you can see

-- 642 is even, i.e. divisible by 2, since it ends in '2'

-- 642 is divisible by 3, since its digits add up to (6+4+2) = 12 which is divisible by 3.

Just looking at 642, you already know of at least 2 factors that it has besides '1' and itself,
so it can't be a Prime number.