Answer:
30 liters of 50% acid solution!
Explanation:
x-liters of 50% acid solution
We need a total of 60*0.6 liters of acid
*36 liters of acid
0.5x+0.7(60-x)=36 (liters of acid)
Expand
0.5x+42-0.7x=36
0.2x=6
x=30
(To solve this informally you can think that you need equal amounts of both solutions since 0.6 is the average of 50% and 70% acid solutions)