7. Physical problem. The volume of water increases by $10 \%$ when it freezes. By what percentage does the volume of ice decrease when it melts?
7. If the volume of water increases by $\frac{1}{10}$ (i.e., becomes $\frac{11}{10}$) when it freezes, then when the ice melts, the volume decreases by $\frac{1}{11}$ (from $\frac{11}{10}$!), which is $9 \frac{1}{11} \%$.