3.3. A package of milk with a volume of 1 liter cost 80 rubles. Recently, in order to save money, the manufacturer reduced the volume of the package to 0.9 liters, and increased the price to 99 rubles. By what percentage did the manufacturer's revenue increase?
Answer: 37.5

Solution. We will calculate by what percentage the cost of 9 liters of milk has increased. Before the increase, 9 liters cost $9 \cdot 80=720$ rubles, and after the increase - $99 \cdot 10=990$ rubles. The increase amounts to 270 rubles for 9 liters. This amount previously cost 720 rubles. This is $\frac{270}{720} \cdot 100 \% = 37.5 \%$