Daniel Boone can paddle his canoe at 15 mph in still water. He is, however, paddling on Plum Creek, which rages along at 5 mph. He paddles downriver for a while, then turns around and paddles back upstream to his starting point. What is his average speed for the entire trip?
We make our trusty table. Note Daniel’s rates are 15 + 5 and 15 – 5.
| d = | r | t | |
| down | d | 20 | d/20 |
| up | d | 10 | d/10 |
| total | 2d | r | d/20 + d/10 |
We want r:
