127. We collected 100 kg of mushrooms. It turned out that their moisture content was $99 \%$. When the mushrooms were dried, the moisture content decreased to $98 \%$. What was the mass of these mushrooms after drying?
Solution. According to the condition, in 100 kg of mushrooms, there is 1 kg of dry matter $(100-0.99 \cdot 100=1)$. Since the mass of dry matter in the total mass of mushrooms is constant (1 kg) and after drying it became $2 \%(100-98=2)$, the mass of mushrooms after drying became 50 kg (if $2 \%-1$ kg, then $100 \%-50$ kg $).$

Answer: 50 kg.

Remark. It is advisable to solve this problem simultaneously with problem 1188 from [1].

$\triangle$ 128. In how many ways can a list of 9 students be compiled?

Answer: 9 !

$\triangle$ 129. There are 35 students in the 6th grade. They exchanged photographs with each other. How many photographs were distributed in total? Answer: $35 \cdot 34=1190$.