7.2 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, part 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 weight of the solution was $500-300=200$ (g)., and the amount of water was $0.99 \cdot 200=198$ (g.), and thus, the substance was $200-198=2$ (g.). After the evaporation of water, 2 g of the substance make up $100 \% -98 \% =2 \%$ of the weight of the solution, so the entire solution weighs 100 g, and together with the glass 400 g.