121. Find all values of the digit $a$, if the number $\overline{875 a}$ is divisible by 6.
$\triangle$ Since this number is divisible by 6, it is divisible by 2 and 3, and vice versa.

Let's apply the divisibility rule for 3. For this, we will find the sum of the digits of the number:

$$
8+7+5+a=20+a
$$

From the divisibility of $(20+a): 3$ it follows that the digit $a$ is 1, 4, or 7. But according to the divisibility rule for 2, this digit must be even, so only $a=4$ fits.

Answer: 4.