Question 1201492
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A description of the standard setup for solving the problem using formal algebra:<br>
You are mixing 5 liters that is 8% weed killer to an unknown number of liters x that is 0% weed killer, resulting in (5+x) liters that is 5% weed killer:<br>
{{{.08(5)+0(x)=.05(5+x)}}}
{{{.4=.25+.05x}}}
{{{.15=.05x}}}
{{{x=.15/.05=15/5=3}}}<br>
ANSWER: 3 liters<br>
If formal algebra is not required, here is a fast and easy informal way to solve the problem.<br>
You are mixing 0% weed killer and 8% weed killer to get 5% weed killer.
5% is 5/8 of the way from 0% to 8%.  (Picture the three percentages on a number line, if it helps.)
That means 5/8 of the mixture is the 8% weed killer.
So 5/8 of the mixture is the 5 liters of 8% weed killer that you started with; that means 3/8 of the mixture is the water you added.
Since 5/8 of the mixture is 5 liters, 3/8 of the mixture is 3 liters.<br>
ANSWER: 3 liters<br>