This one is courtesy of frequent commenter, wst. It’s really a PoSWW, but the ideas may be sufficiently unfamiliar for it to be worth encouraging WitCHlike discussion. It comes from the Cambridge textbook General Mathematics 1&2.
If nothing else, people can enjoy a great song.
This one puzzled us when we gave the 2021 Further Mathematics Exams a three-minute scan, but we didn’t bother then to think, um, further. Now, however, since graph theory is likely a mandatory part of Specialist 1&2, and thus also, at least technically, a mandatory part of Specialist 3&4, we have thought a little harder. So, after discussions with John Friend and with a colleague who we shall refer to as Professor Combo, here we are.