The radiator in a certain make of car needs to contain 20 liters of 40% antifreeze. The radiator now contains 20 liters of 20% antifreeze. How many liters of this solution must be drained and replaced with 100% antifreeze to get the desired strength?

Answer 1

20 litres of 40% contains 20*0.4 = 8 litres of 100% antifreeze.
If we drain x litres and replace with 100% antifreeze, then
0.4(20-x)+1.0x = 8
solve for x.

Answer 2

(4x-12)/(x²+4x+3)*(x²-1)/(x-3)