Solution. Let $a, b$, and $c$ be the three digits that Nikola has thought of. From these three digits, nine two-digit numbers can be written: $\overline{a a}, \overline{b b}$, $\overline{c c}, \overline{a b}, \overline{b a}, \overline{b c}, \overline{c b}, \overline{a c}, \overline{c a}$. Their sum is:

$$
\begin{aligned}
231= & (10 a+a)+(10 b+b)+(10 c+c)+(10 a+b)+(10 b+a)+ \\
& +(10 b+c)+(10 c+b)+(10 a+c)+(10 c+a) \\
= & 33 a+33 b+33 c \\
= & 33(a+b+c)
\end{aligned}
$$

From this, $a+b+c=7$. Given that the digits $a, b$, and $c$ are distinct, the digits that Nikola has thought of are 1, 2, 4, as there are no other possibilities.
2. Nikola thought of three different digits, different from zero. Martin wrote down all the two-digit numbers using these digits. The sum of all the numbers that Martin wrote down is equal to 231. Which digits did Nikola think of?