Hi,
Let x represent the amount of the 50% solution, (600 - x) would then represent the 20% solution
.20(600L - x) + .50x = .30*600L
.20*600L - .20x + .50x = .30*600L
.30x = .10*600L
x = 200L of 50% solution and 400L of 20% solution are needed
checking our answer
.20*400L + .50*200L = 80L + 100L = 180L = .30*600L