Question 372093
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Hi,
Let x represent the total amount of 50% solution. 
Then(24L-x) would be the amount of the 20% solution.
.50x + .20(24L-x) = .30*24L
solving for x
.50x + .20*24L - .20x = .30*24L
.30x = .10*24L
x = 8L of the 50% solution

checking our answer
(16L of the 20% solution (24-8)
.50*8 + .20*16 = 4 + 3.2 = 7.2= .30*24