How many gallons of a 20% ammonia solution should be mixed with 50 gallons of a 50% ammonia solution to make a 30% ammonia solution

We have 50 gallons of 50% ammonia, to which we will add 20% ammonia until we have a 30% ammonia solution.

50 * .50 + .20x = .30*(50 +x) where x is gallons of 20% ammonia.

25 + .20x = 15 + .30x

10 = .10 x
x = 100 gallons of 30% ammonia.