Question 39779
How many litres of 14% alcohol solution must be mixed with 20L of a 50% solution to get a 30% solution. 

Let "x" be amount of 14% solution.
Then 0.14x liters  of this is alcohol.
Also, 0.50(20L)= 10 liters is alcohol.
EQUATION:
alcohol + alcohol = 0.30(x+20)liters
0.14x + 10 = 0.30x + 6
Multiply through by 100 to get:
14x + 1000 = 30x + 600
-16x = -400
x= 25 liters (this is the amount of 14% solution you need to add)
Cheers,
Stan H.