A chemist has one solution of 20% acid and another of 50% acid. how many millimeters of each are needed to obtain 30 ml of 30% acid?

Answer 1

.20x + .50(30-x) = .30(30)
x=20

So, 20ml of 20% and 10ml of 50%

Note that 30% is 1/3 of the way from 20% to 50%, so 1/3 of the acid is the higher concentration.