7.1 In the glass, there was a solution in which water made up $99 \%$. The glass with the solution was weighed, and the weight turned out to be 500 gr. After that, some of the water evaporated, so that in the end, the proportion of water was 98\%. What will be the weight of the glass with the resulting solution, if the weight of the empty glass is 300 gr.?
Answer: 400 g. 
Indication: Initially, the water was $0.99 \cdot(500-300)=198$ (g), and the substance was $200-198=2$ (g). After the water evaporated, 2 g of the substance make up $100 \%-98 \%=2 \%$ of the solution, so the entire solution weighs 100 g, and together with the glass, it weighs 400 g.