16.2.9 * Let $n$ be an integer. If the tens digit of $n^{2}$ is 7, what is the units digit of $n^{2}$?
Let $n=10 x+y, x, y$ be integers, and $0 \leqslant x, y \leqslant 9$, then
$$
n^{2}=100 x^{2}+20 x y+y^{2}=20 A+y^{2}(A \in \mathbf{N}) \text {. }
$$

Since the tens digit of $20 \cdot A$ is even, the tens digit of $y^{2}$ must be odd, which can only be $y^{2}=16$ or 36. Thus, the units digit of $y^{2}$, which is the units digit of $n^{2}$, is 6.