What is the units digit when $5^4$ is multiplied out?
The units digit of a product depends only on the units digits of the factors. So, to find the units digit of $5^4$, I only need to look at the units digit of $5$ raised to the fourth power. I know that $5$ raised to any power will have a units digit of $5$, since $5 \times 5 = 25$, and $25 \times 5 = 125$, and so on. Therefore, the units digit of $5^4$ is $5$. 