17. Water makes up 80 per cent of fresh mushrooms. However, water makes up only 20 per cent of dried mushrooms. By what percentage does the mass of a fresh mushroom decrease during drying?
A 60
B 70
С 75
D 80
E 85
Solution
C

Let the mass of a fresh mushroom be $20 X$ grams. Since water makes up 80 per cent of this, the mass of water in the fresh mushroom is $16 X$ grams and the mass of the remainder is $4 X$ grams. In a dried mushroom, water makes up only 20 per cent. Therefore, the $4 X$ grams of the remainder is 80 per cent of the dried mushroom and hence the mass of water in the dried mushroom is $X$ grams. Therefore, the percentage decrease in the mass of a fresh mushroom during drying is $\frac{20 X-(4 X+X)}{20 X} \times 100=\frac{15 X}{20 X} \times 100=75$.