It’s elementary my dear Watson
Sherlock Holmes and Dr. Watson were on a motorboat going downstream on the Thames. They passed a raft at Waterloo Bridge. An hour later they turned back and eventually passed the raft again. The raft was now six miles from Waterloo Bridge. Sherlock asked Watson, “do you know what the velocity of the river is?” “That is quite impossible to determine Holmes”, Watson exclaimed. Holmes replied, “it’s elementary my dear Watson”, and then he explained how easy it was to do it.
So, how did he Sherlock do it, and how fast was the Thames?
(Yes, I know Holmes never used that phrase).