3.1. A chocolate bar weighed 250 g and cost 50 rubles. Recently, to save money, the manufacturer reduced the weight of the bar to 200 g, and increased its price to 52 rubles. By what percentage did the manufacturer's revenue increase?
# Answer: 30

Solution. We will calculate by what percentage the cost of one kilogram has increased. Before the increase, 1 kg cost 200 rubles, and after the increase, it costs $52 \cdot 5=260$ rubles. The increase is 60 rubles per kg, which originally cost 200 rubles. This is $\frac{60}{200} \cdot 100 \% = 30 \%$