Question

Explain why numbers with a 5 in the ones place are not prime numbers

Answer 1

If a number has 5 as it's last digit, it's always divisible by 5: multiples of 5 have either 5 or 0 as the last digit.

If a number is divisible by 5, it is not a prime with one exception: of 5 itself. This is from the definition of prime numbers: divisible only by 1 and by itself.