Mikuláš prepared packages for 23 children. In each, he was supposed to put a chocolate weighing 100 grams, a bag of candies weighing 90 grams, wafers weighing 80 grams, and three different ten-gram packets of gum. After wrapping, he put all the packages on the scale and found that they weighed a total of $7 \mathrm{~kg}$. Mikuláš immediately realized that he must have made a mistake. Indeed, upon checking, he found that one and the same item was extra in two of the packages and one package was missing the wafers. What was the extra item in each of the two packages?

(Bednářová)
First, we will determine the weight of one package:

$$
\begin{array}{ll}
\text { chocolate } & \ldots 100 \text{ g} \\
\text { candies } & \ldots 90 \text{ g} \\
\text { wafers } & \ldots 80 \text{ g} \\
\text { chewing gum } & \ldots 3 \times 10=30 \text{ g} \\
\hline \text { in total } & \ldots 300 \text{ grams }
\end{array}
$$

From this, we can easily calculate that 23 packages should weigh $23 \cdot 300 = 6900$ grams. However, the total weight was $7 \text{ kg} = 7000 \text{ g}$, which is 100 grams more. Additionally, in one package, the wafers (weighing 80 grams) were missing. The total difference was 180 grams, or two candies.

In each of the two packages, there were extra candies.

- determination of the total weight (2 points)
- determination of the 180-gram difference (2 points)
- complete solution (6 points)