It is necessary to have a 40% antifreeze solution in the radiator of a certain car. The radiator now has 60 liters of 20% solution. How many liters of this should be drained and replaced with 100% antifreeze to get the desired strength?

Answer 1

let the amount to be drained be x l
then
.2(60-x) + 1.0(x) = .4(60)
2(60-x) + 10x = 4(60)
etc.

I got x = 15