SOLUTION: Write an equation in terms of x that determines how much water should be added to x liters of an 18% solution to reduce the concentration to 15%?

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Question 902516: Write an equation in terms of x that determines how much water should be added to x liters of an 18% solution to reduce the concentration to 15%?
Found 2 solutions by richwmiller, josgarithmetic:
Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
x*0.18=0.15*(x+y)
x is the amount of solution
y is the amount of water to be added.

Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
Percent will be treated as weight per weight.

Let D = density of the salt solution, grams/mL, 18% solution.
Let y = MASS in grams of water to add to get 15% salt.
The reason for the factor 1000 is because I am solving with GRAMS and MILILITERS.

highlight%28%280.18%2A%28x%2A1000%2AD%29%29%2F%28x%2A1000%2AD%2By%29=0.15%29

Algebra Steps---
0.18D%2A1000x=0.15%28D%2A1000x%2By%29
0.15%2A1000%2AD%2Ax%2B0.15y=0.18%2A1000%2AD%2Ax
0.15y=0.18%2A1000Dx-0.15%2A1000Dx
y=%280.18%2A1000Dx-0.15%2A1000Dx%29%2F0.15
y=%280.03%2F0.15%29%281000Dx%29
y=%280.2%291000Dx
highlight%28highlight%28y=200Dx%29%29, GRAMS of water.