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.
By request, we’ve added an excerpt from the follow-up section of the text, on planar graphs.
Frequent commenter SRK has pointed out the following question from the Network module of the 2017 Further Exam 1. We *think* the question is ok enough, but there is definitely room for debate. (VCAA’s intended answer can be found here.)