Question

How much 90 percent vinegar do you have to add to a gallon of 5 percent vinegar to get 20 percent vinegar?

Answer 1

x(90%) + 1(5%) = (x+1)(20%)

.9x + .05 = .2x + .2
.9x -.2x = .2 - .05
.7x = .15

x = .15/.7 = 15/70 = 3/14 gallons

3/14 of a gallon needs to be added. If I did the math in me head correctly :)