Algebra problem Increasing the percentage?

Question

A mechanic has found that a car, with a 16-quart radiator has a 40% antifreeze mixture in the radiator. He has on hand a 70% antifreeze solution. How much of the 40% solution would she have to replace with the 70% solution to get the solution in the radiator up to 50%?

Answer 1

x = the amount of 40% fluid to use
16-x = the amount of 70% fluid to use

Here is the equation you need to use:

.4x + .7(16-x) = .5(16)

Answer 2

.4 *16 +.7*x = (16+x)* .5
6.4 + .7x = 8 + .5x
.2x = 1.6
x = 8 quarts