This puzzle is supposed to be classic, but for some reason I haven’t heard about it, and there is a possibility that neither have you. Find the length of the red line, using only school-level math:
This is one of those puzzles that seems difficult, but has a simple, retrospectively-obvious solution. Don’t worry, though: you will probably find the answer in a matter of minutes, if not faster. It’s pretty easy, as far as these things go.