Math
posted by Daniel .
How many liters of water should be added to 20 liters of 40% solution of acid to obtain 30% solution?

Acid = 0.4 * 20 = 8 liters.
Water = 0.6 * 20 = 12 lites.
A / (A + W) = 0.30,
8 / (8 + W) = 0.30,
2.4 + 0.3W = 8,
0.3W = 8  2.4 = 5.6,
W = 18.7 Liters.
18.7  12 = 6.7 Liters of water added. 
x = liters of water
.40(20) = .30(x + 20)
8 = .30x + 6
8  6 = .30x + 6  6
2 = .30x
2 / .30 = .30x / .30
6.666 = x (which is also 6 2/3 as a fraction)
6.7 ~ x
6.7 liters of water is needed.
or
Six and twothird liters of water is need to obtain a 30% acid solution.