Per the Guardian, but I saw it at Bookninja. The funniest thing in the article, actually, is the strenuously overthought commentary by the classics scholar who found the book:
But she queried whether we are finding the same things funny as the Romans would have done. Telling a joke to one of her graduate classes, in which an absent-minded professor is asked by a friend to bring back two 15-year-old slave boys from his trip abroad, and replies “fine, and if I can’t find two 15-year-olds I will bring you one 30-year-old,” she found they “chortled no end”.
“They thought it was a sex joke, equivalent to someone being asked for two 30-year-old women, and being told okay, I’ll bring you one 60-year-old. But I suspect it’s a joke about numbers – are numbers real? If so two 15-year-olds should be like one 30-year-old – it’s about the strange unnaturalness of the number system.”
Professor Beard, I’m going to say it’s probably a sex joke. We are talking about Romans here.