9. Seawater contains $5 \%$ salt. How many kilograms of fresh water need to be added to 40 kg of seawater to make the salt content $2 \%$?
9. In 40 kg of seawater, there is $40 \cdot 0.05=2$ (kg) of salt, which in the new solution constitutes $2\%$, therefore, the solution should be $2: 0.02=100$ (kg). Answer: 60 kg of fresh water should be added.