13.008. Seawater contains $5 \%$ salt by mass. How much fresh water needs to be added to 30 kg of seawater to make the salt concentration $1.5 \%$?
## Solution.

Let $x$ kg of salt in 30 kg of seawater. $x$ kg is $5 \%$. Hence, $x=1.5$ kg, $30 \text{kg}-100 \%$. In the diluted seawater: 1.5 kg of salt is $1.5 \%$, $a$ kg is $100 \%$, from which $a=100$ kg of diluted seawater. Therefore, we need to add $100-30=70$ kg of fresh water.

Answer: $70 \text{kg}$.