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A 10 kg salt solution originally $$4 \%$$ by weight NaCl in water is evaporated until the concentration is 5% by weight NaCl. What percentage of water in the original solution is evaporated?

Solution

 Water $$96%$$ - $$100\%$$ $$95\%$$ Salt $$4%$$ - $$0\%$$ $$5\%$$ -- $$100\text{kg}$$ - $$x$$ $$100-x$$

$$0.96(100) - x = 0.95 (100-x)$$

$$x= 20 \text{kg}$$

Since there are 96 kg water and 20 kg evaporated water, thus the percentage of the evaporated water is $$\frac{20}{96} (100) = 20.83\%$$