Math equation help? need help on math equation

How would I solve this? How many liters of 20% alcohol solution should be added to 40 liters of a 50% alcohol solution to make 30% solution?

x = liters 30% solution
40(.5) + x(.2) = (40+x)(.3)
20 + 0.2x = 12 + 0.3x
8 = 0.1x
x = 80 liters
check work
40(.5) + 80(.2) = (40+80) * (.3)
20 + 16 = 120 * (.3)
36 = 36

I'm guessing that u would add 20% with 40L and ull get 60% then subtract it bye 30 2 get 30%
I'm 10 years old check with ur teacher 2 make sure