Question

A solution containing 30% insecticide is to be mixed with a solution containing 50% insecticide to make 200 L of a solutions containing 42% insecticide. How much of each solution should be used?

Answer 1

Let x L of solution 1 is used 
and 2nd solution is (200-x)L

According to question,
30% of x + 50% of (200-x) = 42% of 200L
0.3x + 0.5(200-x) = 84
0.3x - 0.5x + 100= 84
0.2x = 16
x=80

Therfore 80L of solution 1 and 120L of solution 2 is mixed.