How much of a 60% acid solution must be mixed with 50 gallons of a 24% acid solution to obtain a solution that is 50% acid?

My teacher taught me a simple equation for these types of problems:
(percent of solution × amount of solution) + (percent of solution × amount of solution) = (percent of solution × total amount of solution) 
in simple terms:    %amount + %amount = %total amount
We know don't know how much gallons of 60% solution we need so it will be represented as x. We dont know the total amount either so it would have to be both solutions added together so 50 + x. Now lets solve:
(0.60)(x) + (0.24)(50) = (0.50)(50 + x)            simplify/ distribute
0.6x + 12 = 25 +0.5x                  get all xs on one side and numbers on other
0.1x + 12 = 25
0.1x = 13    divide both sides by 0.1 to find x
x = 130
You need 130 gallons of 60% solution.