To test a number for 8, which is 2 cubed, how many ending digits need checking?

1. For a power of 2 such as 2 to the n, look at the last n digits. 2. Here 8 = 2 cubed, so n = 3 and the last three digits are checked. 3. Two digits is the rule for 4, and one digit for 2. _Source: Wikipedia, Divisibility rule, section Composite divisors_