#### Answer

10 liters of alcohol must be added in the solution.

#### Work Step by Step

Let p represent the amount of alcohol to be added.
After p is added, we obtain a 60% solution.
We use the following guideline to solve this problem:
Amount of pure alcohol in original solution + Amount of pure alcohol to be added = Amount of pure alcohol in final solution.
So: 20 $\times$ 40% + p = 60% (20 + p)
$20 \times .4 +p =.6(20+p)$
8 + p = 12 + $\frac{3p}{5}$
4 = p - $\frac{3p}{5}$
4 = $\frac{5p}{5}$ - $\frac{3p}{5}$
4 = $\frac{2p}{5}$
Multiply both sides by 5.
20 = 2p
Divide both sides by 2:
p = 10
10 liters of alcohol must be added in the solution.