Pat wrote on the board the example:

$$
589+544+80=2013 .
$$

Mat wanted to correct the example so that both sides would actually be equal, and he searched for an unknown number which he then added to the first addend on the left side, subtracted from the second addend, and multiplied the third addend by. After performing these operations, the example was numerically correct. What number did Mat find?

(L. Hozová)
By adding an unknown number to the first addend and subtracting the same number from the second addend on the left side, the sum of these two numbers will not change and is equal to $589+544=$ $=1133$. This intermediate sum is $2013-1133=880$ less than the number on the right side of the equation. Therefore, the product of 80 and the unknown number Mat found should be equal to 880. The number Mat found was $880: 80=11$.

Evaluation. 2 points for recognizing that the first two operations have no effect on the result; 2 points for expressing the difference 2013 - $1133=880$ and explaining its significance; 2 points for expressing the unknown number.