Answer
12 minutes
Work Step by Step
We add the rates of the two machines to find their overall rate:
$$ \frac{1}{20}+\frac{1}{30}\\ \frac{3}{60}+\frac{2}{60}\\ \frac{5}{60}\\ \frac{1}{12}$$
Thus, taking the reciprocal, it would take them 12 minutes to do it together.
