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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In words:
(amount of weed killer) / (weed killer + H2O) = % solution
I start with 3 oz of 80% solution. In this 3 oz, there is
oz of weed killer.
Let = amount of water to be added
I'm not adding weed killer, just water, so

Multiply both sides by




9 oz of water must be added to get a 20% solution
check:



OK