4 ms·

why did you write this?

• ColinWright 12y ago
  It's a puzzle, and a good one. There are puzzles where the challenge is to work out what the insight is needed to see the solution, and there are puzzles where the challenge is to work out exactly what calculation is needed (and then to do it). In this case the puzzle is to find the error in the reasoning. Other examples of this include the two-envelope problem, the unexpected hanging, and the case of the missing dollar. Sometimes puzzles like this reveal deep insights into the subject, sometimes they point out that "obvious" things aren't obvious, and sometimes they reveal weaknesses in our understanding or reasoning. This is - for some students - an excellent introduction to why we need to be careful when dealing with certain types of mathematical manipulation.