It is necessary to have a 40% antifreeze solution in the radiator of a certain car. the radiator now has 70 liters of 20% solution. how many liters of this should be drained and replaced with 100% antifreeze to get the desired strength?

The answer is : 17.5 liters drained and replaced by 17.5 liters of 100% solution.

x = amount drained and replaced

(70-x) = remaining amount of 20% solution

.20 (70-x) + 1.00 (x) =.40 (70)

14 -.2x + 1x = 28

1x -.2x = 28 - 14

.8x = 14

x = 14/.8

x = 17,5 (17.5 liters drained and replaced by 17.5 liters of 100% solution)