Solving a percent mixture problem using a linear equation

Question

In the lab, Lena has two solutions that contain alcohol and is mixing them with each other. She uses 3

 times as much Solution A as Solution B. Solution A is 16%

 alcohol and Solution B is 20%  alcohol. How many milliliters of Solution B does she use, if the resulting mixture has 136 milliliters of pure alcohol? 

Answer 1

a = 3b

0.16a + 0.2b = 136

Substitute!

0.16(3b) + 0.2b = 136

0.48b + 0.2b = 136

0.68b = 136

b = 200 ml