Question 74505
When the tank is filled with 40% solution, there will be 32 liters of chemical in the solution (40% of 80 liters or .4 X 80).  y liters of 30% solution must be drained and replaced with 70% solution.


(chemical left in tank) + (chemical added) = 32 liters


{(80-y) X .30} + (y X .70) = 32


(24-.3y) + .7y = 32


.4y = 8


y = 20 liters