How to calculate group work?

The formula for group work is
$\text{group work}=\frac{1}{{\displaystyle \frac{1}{A}}+{\displaystyle \frac{1}{B}}}$
where A is work done alone by A in hours, and B is work done alone by B in hours.

Use the calculator above to calculate how many hours it will take for A and B to finish the job together.