1. Sea water contains $5 \%$ salt. How many kilograms of fresh water need to be mixed with $40 \mathrm{~kg}$ of sea water to obtain a mixture that contains $2 \%$ salt?
Solution. In $40 \mathrm{~kg}$ of seawater, there is $40 \cdot 0.05=2 \mathrm{~kg}$ of salt. If we add $60 \mathrm{~kg}$ of fresh water, we will have $100 \mathrm{~kg}$ of mixture with $2 \mathrm{~kg}$ of salt, which is $2 \%$ salt.