SPOILER ALERT!! Have you heard of Cheryl's birthday puzzle? How good are you - or you children - at maths? That question has caused panic amongst parents in Singapore after a devilishly complex puzzle was published by the organisers of a maths competition. The test challenged mathematicians to work out the date of a girl's birthday based on a small number of clues. But the testers mistakenly said the fiendish calculation was appropriate for primary school pupils - when in fact, it was intended for elite fifteen to sixteen year olds. So how difficult was it? Mathematician and author, Alex Bellos, took me through it.