Let q be the number of quarts of pure antifreeze that needs to be added to get the desired solution.
8 quarts of 40% solution contains 0.40 × 8 = 3.2 quarts of antifreeze.
The new solution would have a total volume of 8 + q quarts, and it would contain a total amount of 3.2 + q quarts of antifreeze. You want to end up with a concentration of 60% antifreeze, which means
(3.2 + q) / (8 + q) = 0.60
Solve for q :
3.2 + q = 0.60 (8 + q)
3.2 + q = 4.8 + 0.6q
0.4q = 1.6
q = 4
