Teaser 2974: Flockbuster

From The Sunday Times, 22nd September 2019 [link]

Wu, Xi, Yo and Ze had different two-figure numbers of sheep and kept them in a walled field divided by fences into a fold each. Maths whizz, Wu, with the largest flock, noticed that together her flock and Ze’s equalled Xi’s and Yo’s combined; and that, as a fraction, the ratio of Yo’s flock to Xi’s had consecutive upper and lower numbers (e.g. 3/4), whereas her flock to Xi’s ratio had those numbers swapped over (e.g. 4/3).

Overnight, storm-damaged fences led to the same number of sheep in each fold. Wu’s old maths teacher, told just this number and the above relationships, couldn’t be certain how many sheep Wu owned (which would have been true, also, if he’d been told either fraction instead).

How many sheep did Wu own?