Brain-Teaser 502

From The Sunday Times, 17th January 1971 [link]

A publisher friend of mine told me the following improbable story. He dreamt that on his shelves he had a collection of twelve books of the same edition of the same title. The curious thing about these books was this:

The book, as published, ran to 300 pages, but eleven of the books on his shelves had each been bound with duplicate leaves inserted. The number of leaves thus duplicated was different in each volume, all except two being even numbers. They went in ascending order from the book on the left (Volume I), the highest being twenty extra leaves in Volume XI.

The end book on the right (Volume XII) had 200 pages missing. By a strange coincidence, the duplicated sections from the other volumes exactly filled these gaps. In the reconstituted Volume XII the middle pages (143 to 158) came from the middle volume of the faulty books, while half the duplicated pages came before these and an equal number in the latter half of the volume.

How many pages were duplicated in Volume IV?