G9.1 If $(1995)^{a}+(1996)^{a}+(1997)^{a}$ is divisible by 10 , find the least possible integral value of $a$.
The unit digit of $(1995)^{a}+(1996)^{a}+(1997)^{a}$ is 0 .
For any positive integral value of $a$, the unit digit of $1995^{a}$ is 5 , the unit digit of $1996^{a}$ is 6 .
The unit digit of $1997^{a}$ repeats in the pattern $7,9,3,1, \ldots$
$$
5+6+9=20
$$

So the least possible integral value of $a$ is 2 .