Question

Solution a is 80 percent acid, and solution b is 30 percent acid. How much of each solution is needed to make 200 ml of a solution that is 45 percent acid?

Answer 1

add up the acid amounts. The sum of the parts must equal the whole.
If there are x ml of 80% acid, then the rest (200-x) is 30%. So,

.80x + .30(200-x) = .45*200

Now just crank it out.