## 163. Math Puzzle $12 / 78$

A retention basin has a capacity of 0.5 million $\mathrm{m}^{3}$ of water. Even during low water, 20 $\mathrm{m}^{3}$ of water flow out per hour.

After a storm, a flood with an average of 120 $\mathrm{m}^{3}$ of water per hour flows in for ten hours.

By what percentage will the basin be filled?
The retention basin is filled by 0.2 percent, because after ten hours,

$$
\begin{array}{lr}
\text { the inflow } 10 \cdot 120 \mathrm{~m}^{3} & =1200 \mathrm{~m}^{3} \\
\text { the outflow } 10 \cdot 20 \mathrm{~m}^{3} & =200 \mathrm{~m}^{3} \\
\hline \text { the filling thus } & 1000 \mathrm{~m}^{3}
\end{array}
$$

This is $1 / 500$ of the capacity, which is 0.2 percent.