Question

How many liters of 80% alcohol solution and 30% alcohol solution must be mixed to obtain 15 liters of 50% alcohol solution?

Answer 1

set up a table (multiply across and add down - don't add the middle column). Use the equation created by the "Total" row to solve for the variable.

               Quantity  (times)      %        equals        Total

Sol'n 1:          x            *           .80             =            (x)(.8)

Sol'n 2:      15 - x        *           .30             =          (15 - x)(.3)

Total:          15            *          .50              =          .8x + .3(15 - x)

                                15(.5)                        =         .8x + 4.5 - .3x

7.5 = .5x + 4.5

3.0 = .5x

6 = x

Answer: 6 liters of 80% and 9 liters of 30%