"丨"既數學問題

n^5 - n
= n (n^4 -1)
= n (n^2 - 1) (n^2 +1)
= n (n+1) (n-1) (n^2 +1)

For
n=2, n^5 - n = 2x3x1x5 is divided by 2 and 5
n=3, n^5 - n = 3x4x2x10 is divided by 2 and 5
n=4, n^5 - n = 4x5x3x17 is divided by 2 and 5
n=5, n^5 - n = 5x6x4x26 is divided by 2 and 5
n=6, n^5 - n = 6x7x5x37 is divided by 2 and 5
n=7, n^5 - n = 7x8x6x50 is divided by 2 and 5
n=8, n^5 - n = 8x9x7x65 is divided by 2 and 5
n=9, n^5 - n = 9x10x8x82 is divided by 2 and 5
n=10, n^5 - n = 10x11x9x101 is divided by 2 and 5
n=11, n^5 - n = 11x12x10x122 is divided by 2 and 5
n=12, n^5 - n = 12x13x11x145 is divided by 2 and 5
n=13, n^5 - n = 13x14x12x170 is divided by 2 and 5
.
.
.
You can see all last digit numbers (0 to 9) can be divided by 2 and 5.
Therefore, 10│n5-n when n is a natural number.