3.26 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?
3.26 In the cellulose mass, there is $0.85 \cdot 500 = 425 \mathrm{kg}$ 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: $200 \mathrm{kg}$.