13.002. How many kilograms of water need to be evaporated from 0.5 t of cellulose mass containing $85 \%$ water to obtain a mass with $75 \%$ water content?
Solution.

In the cellulose mass, there is $0.85 \cdot 500=425$ kg of water. Let $x$ kg of water be evaporated; then we get $425-x=0.75(500-x)$, from which $x=200(\mathrm{kg})$.

Answer: $200 \mathrm{kg}$.