To determine the units digit of a number raised to a power, we only need to look at the units digit of the base number. The units digit of 77 is 7 . Let's see what happens when we raise 7 to different powers: 71=7 72=49 73=343 74=2401 Notice a pattern in the units digits: 7,9,3,1. This pattern repeats. The units digit of any power of 7 will be one of these four digits. Since the units digit cycles through four values, the probability that the units digit of 77n is 1 is 1 out of 4 .