Explain how to solve:
A chemist has one solution containing 30% insecticide and another solution containing 50% insecticide. How much of each solution should the chemist mix to get 200 L of a 42% insecticide?
.3 x + .5 y = .42(x+y)
and
x+y = 200
so y = (200-x)
.3 x + .5 (200-x) = 84
etc