I think this turned out to be an even trickier problem than Alex Bellos thought.
“Tricky triangle This one was sent in by a reader, aged 85, who first saw it in 1960. He is a roboticist who passed through Harvard, Princeton, Stanford and IBM. He says it is his favourite puzzle. ‘I’ve given this puzzle to perhaps 100 people. Over 80% have no idea how to solve it.’ What is the length of AD, the dashed line?”
See Tricky Triangle for solutions.