Achemist needs 9 liters of a 50% salt solution. all she has is a 20% salt solution and a 70% salt solution. how many liters of the 20% solution does she need to mix with the 70% solution to get what she needs

We have 20% and 70% salt solutions.
We need 9 liters of 50% salt solution.
"t" is liters of twenty % solution "s" is liters of seventy % solution

A) t + s = 9
B) .2t + .7s = (.5 * 9)
Multiplying A) by -.2
A) -.2t + -.2s = -1.8 Adding that to B)
B) .2t + .7s = (.5 * 9)
.5s = 2.7
s = 5.4 liters of 70% salt

A) t + 5.4 = 9

t = 3.6 liters of 20% salt

Source:
http://www.1728.org/mixture.htm