The most common initial incorrect answer is 21. That said, some “contestants” came back at a later time, even the next day or week with the correct answer, which led author John Driscoll to wonder why they couldn’t get the answer right first time. 

His book A Hen and a Half was the result. The acronym of the book title - Ahaah - resembles and exaggerates the equivalent Eureka exclamation – Aha! - and intends to replicate the moment as the reader realises the missing and often, the sometimes obvious!

You can get the full answer to this conundrum in the book – or we can email it you here. [link to be added]

Have a go at some of the other puzzles on this site before trying again. Or check out the book here.