There are four fences, which are geometrical identical (same heigth and same length). The following four people need the specified time to paint it:

* Bob needs 2 hours.
* Simon needs 3 hours.
* Tommy needs 5 hours.
* Johnny needs 8 hours.

How long does it take, if they paint one mentioned fence together?

Bonus: what is the special electrotechnical approach to this issue?