a chemist has 12% hydrochloric acid+24% hydrochloric. How many of each should be 16 liters of 21%hydrochloric?

12% needed --- x

24% needed --- 16-x

solve:
.12x+ .24(16-x) = .21(16
times 100
12x + 24(16-x) = 21(16)
take over....