At the camp, the scouts were weighing themselves on an old-fashioned scale. The leader warned them that the scale did not weigh correctly, but the difference between the actual and measured weight was always the same. The scale showed $30 \mathrm{~kg}$ for Misha, $28 \mathrm{~kg}$ for Emil, but when they both stood on the scale, it showed $56 \mathrm{~kg}$. What was the actual weight of Misha and Emil?

(M. Volfová)
In Míša's result of $30 \mathrm{~kg}$, one weighing error was included, just as in Emil's result of $28 \mathrm{~kg}$, which also included the same weighing error.

In the sum $30+28=58(\mathrm{~kg})$, two weighing errors are therefore included. However, when Míša and Emil weighed themselves together, the result of $56 \mathrm{~kg}$ included only one weighing error. The difference $58-56=2(\mathrm{~kg})$ thus represents exactly one weighing error.

Míša's actual weight was $30-2=28(\mathrm{~kg})$, and Emil weighed $28-2=26(\mathrm{~kg})$.

Evaluation. 4 points for deriving the weighing error; 1 point each for Míša's and Emil's weights.