The pure salt is unchanged, 500 * 16% = 80g, and then the concentration is increased to 20%. Using the formula solute/concentration = solution, it is found that the latter solution is 80/20% = 400g, and the reduced solution 100g is the mass of evaporated water.
Comprehensive formula: 500-(500 *16%)/20% =100g.
Solution 2 (proportional solution):
At first, the ratio of pure salt to water was 80: 420 (calculated).
Later, the ratio of pure salt to water was 20: 80, and the front and back terms of the ratio were multiplied by 4 at the same time.
Later, when the ratio of pure salt to water was 80: 320, it was easy to see that water changed from 420 to 320.
So the evaporated water is 420-320 = 100g.
Solution 3 (Equation Solution):
Assuming that the evaporated water is x grams, according to the mass of pure salt before and after evaporation
500 * 16% = 20%(500 - x)
The solution is x = 100.