Question 744420
The problem states

What quantity of 60 percent acid solution must be mixed with a 30 percent solution to produce 630 mL of a 50 percent solution?

let x = amount of .60 solution
then 630 - x = .30 solution
and .50*630 = 315 ml of alcohol

then

.6x + .3(630 - x) = 315
.3x = 315 - 189 
.3x = 126   and

x = 420

420ml of 60% solution and 
210ml pf 30% solution