3.4. A package of milk with a volume of 1 liter cost 60 rubles. Recently, in order to save money, the manufacturer reduced the volume of the package to 0.9 liters, and increased its price to 81 rubles. By what percentage did the manufacturer's revenue increase?
# Answer: 50

Solution. We will calculate by what percentage the cost of 9 liters of milk has increased. Before the increase, 9 liters cost $9 \cdot 60=540$ rubles, and after the increase, $-81 \cdot 10=810$ rubles. The increase amounts to 270 rubles for 9 liters. This amount previously cost 540 rubles. This is $\frac{270}{540} \cdot 100 \% = 50 \%$