Can someone at least explain this?

a sample of 80 high school seniors, 45% of which are female, were surveyed as to whether or not they are employed outside of school. 25% of the boys surveyed said they are not employed. how many boys are employed?

(n^3 - n)

= n(n-1)(n+1)

since it is divisible by 3 and 2 ,because it is a multiple 3 consecutive number

if, n-1 is odd then n+1 is also odd, then it is a multiple of 9

but if, n-1 is even then n+1 is also even, then it is a multiple of 4

so, largest multiple = max(9*2, 4*3) = max(18, 12) = 18