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?