Find the units digit of $17^{2021}$.
The units digits of powers of 17 cycle: $7,9,3,1,7,9,3,1, \ldots$, so the units digit of $17^{n}$ is 1 whenever $n$ is a multiple of 4 . Since 2020 is a multiple of 4 , $17^{2020}$ has units digit 1 , so $17^{2021}$ has units digit 7 .