Question

Charlie mixes 15 liters of 17% acid solution with a 29% acid solution, which results in 23% acid solution.

Answer 1

(15(.17)+.29x)/(15+x)=.23

2.55+.29x=3.45+.23x

.06x=.9

x=15

So 15L of the 29% solution was added to the 15L of 17% solution to make  

30L of 23% solution.