During a chemistry class, David was asked to add pure acid to 6 liters of 45% acid solution, to get a solution which is 80% acid. How much water should he add?

10.5 liters

Step-by-step explanation:

In this question you are mixing 6liters of 45% acid solution to get a an acid solution of 80%.

This can be written as;

(0.45 * 6) + (1.00 * x) = 0.80 (6+x)

2.7 + x = 4.8 + 0.80x

x-0.80x=4.8-2.7

0.20x=2.1

x=2.1/0.20 = 10.5 liters