SOLUTION: I have 3 oz. of an 80% solution of weed killer. How much pure water must i add to get a 20% solution?

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Question 170500: I have 3 oz. of an 80% solution of weed killer. How much pure water must i add to get a 20% solution?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
In words:
(amount of weed killer) / (weed killer + H2O) = % solution
I start with 3 oz of 80% solution. In this 3 oz, there is
.8%2A3+=+2.4 oz of weed killer.
Let x= amount of water to be added
I'm not adding weed killer, just water, so
2.4+%2F+%28x+%2B+3%29+=+.2
Multiply both sides by x+%2B+3
2.4+=+.2%2A%28x+%2B+3%29
2.4+=+.2x+%2B+.6
.2x+=+1.8
x+=+9
9 oz of water must be added to get a 20% solution
check:
2.4+%2F%289+%2B+3%29+=+.2
2.4%2F12+=+.2
.2+=+.2
OK