SOLUTION: How many liters of a 9% solution of salt should be added to a 21% solution in order to obtain 540 liters of a 15% solution?

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Question 703984: How many liters of a 9% solution of salt should be added to a 21% solution in order to obtain 540 liters of a 15% solution?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Notice that 15 is exactly in the middle of 9 and 21. EXACTLY. Maybe half of 540 liters of each solution?

x and y are liters volume of the 9% and the 21% respectively, to use.
x+y=540.

Account for the percntages,
%28x%2A0.09%2By%2A0.21%29%2F540=0.15
%28x%2A0.09%2By%2A0.21%29=0.15%2A540
9%2Ax%2B21%2Ay=15%2A540
Substitute x=540-y into this equation,
9%2A%28540-y%29%2B21%2Ay=15%2A540
Continue to find y...
12y=15%2A540-9%2A540
y=540%2A%286%2F12%29
y=270 liters