In the quinary (base-5) number system, each digit represents powers of 5, similar to how the decimal system (base-10) works with powers of 10.
The number "13" in quinary can be expanded into decimal as follows:
1 x 5^1 + 3 x 5^0
= 1 x 5 + 3 x 1
= 5 + 3
= 8
In the decimal system, 8 is divisible by 4 (since 8 / 2 = 4 with no remainder).
Since a prime number is a natural number greater than 1 that has no divisors other than 1 and itself, and 8 is divisible by 4 (and also by 2), it is not a prime number.