Amechanic needs a radiator to have 50% antifreeze solution. the radiator currently is filled with 4 gallons of 15% antifreeze solution. how much antifreeze mixture should be drained from the car if the mechanic replaces it with pure antifreeze.

let x=gallons of current mixture to be drained and replaced with pure antifreeze.
4-x=gallons of current mixture remaining in the car.

0.15(4-x)+1.00x=0.50 x 4
0.6-.15x+x=2
0.85x=1.4
x=1.4/0.85 =1.65 gal

Thus, 1.65 gallons of current mixture to be drained and replaced with pure antifreeze.