Answer
15 centiliters of distilled water must be added.
Work Step by Step
Let X represent the amount of water to be added.
After X is added, we obtain a 20% acid solution.
A 20% acid solution has 80% water in it.
A 50% acid solution has 50% water in it.
We use the following guideline to solve this problem:
Amount of water in original solution + Amount of water to be added = Amount of water in final solution.
50% $\times$ 10 + X = 80% $\times$ (10 + X)
$.5\times 10 +X=.8 \times (10+X)$
5 + X = $\frac{4}{5}$ $\times$ (10 + X)
5 + X = 8 + $\frac{4X}{5}$
3 = $\frac{5X}{5}$ - $\frac{4X}{5}$
Multiply both sides by 5
X = 15
15 centiliters of distilled water must be added.
