Expert: Richard J. Raridon - 4/8/2011

Question
a radiator contains 8 gallons of a solution which is 20% alcohol. how much of the solution must be drained off and replaced by pure alcohol to obtain a solution containing 30% alcohol.

Answer
Original solution contains 1.6 gallons of EtOH and 6.4 gallons of H2O
The final solution contains 2.4 gallons of EtOH and 5.6 gallons of H2O
By removing 1 gallon of original solution you are removing
0.2 gallons of EtOH and 0.8 gallons of water and replacing it
by 1.0 gallon of EtOH.  That will give you the final solution.