A puzzle by Matt Parker of standupmaths:
The standard paper size A4 has dimensions in the ratio . Hold a piece of A4 paper horizontally, as shown, and fold down the top left corner to meet the other side, creating fold AB, as if you were going to make a paper square. Then fold down the top right corner to meet the edge of this 1 × 1 square (making fold BC).
The perimeter of the original sheet was . What is the perimeter of the folded shape (the quadrilateral ABCD above)?
I’ll honor Matt’s request not to reveal the answer, but here’s a clue: The shape ABCD is a kite. See Matt’s video for a more visual explanation and a non-spoilery way to tell whether you have the right answer.
(Thanks to Dave Lawrence for the tip and the diagram.)