Answer: $1+9+1=11$.

Comment (how the answer could be found): On the right, there is a two-digit number with two identical digits. Since the largest sum of three digits is $9+9+9=27$, the number on the right is less than 30, i.e., it is 11 or 22. If it is 11, we get: $1+\triangle+1=11$, from which $\triangle=9$. If it is 22, then $2+\triangle+2=22$, but then $\triangle=18$, which does not fit, as it must be a digit. Therefore, the answer is unique.

## Grading Criteria.

- Correct example (no justification required) - 7 points.
- Example where a two-digit number is written in the triangle, and the same digit is written in the squares, for example, $3+27+3=3 \sqrt{3}-1$ - 1 point.
1. In the addition example: $\square+\triangle+\square=\square \square$, fill in the same digit in each square and a different digit in the triangle so that the example becomes correct.