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.