## 170. Math Puzzle $7 / 79$

Michael prepares 3 l of a 7.2 percent saline solution, meaning that in every $100 \mathrm{~g}$ of the solution, there are exactly 7.2 g of salt. By boiling this solution, so much water evaporates that exactly $2.4 \mathrm{l}$ of the concentrated solution remains.

What is the percentage concentration of the resulting solution?
The 31 contains $216 \mathrm{~g}$ of table salt: $7.2 \cdot \frac{3000}{100}=216$.

After the water evaporated, the solution was 9 percent, because

$$
\frac{216}{2400} \cdot 100=9
$$