An Odd Number

An odd number to the fourth power will always be one more than a multiple of eighty, unless the odd number is divisible by five.

Examples:

1^4 = 1, 1 - 1 = 0, 0 is a multiple of eighty

3^4 = 81, 81 - 1 = 80, 80 is a multiple of eighty

7^4 = 2401, 2401 - 1 = 2400, 2400 is a multiple of eighty

9^4 = 6561, 6561 - 1 = 6560, 6560 is a multiple of eighty