That depends on whether the question conveys Billy having read 25 pages per day for book one, or if he read everyday but the book has 25 pages.

IMO better wording would be: Billy read 2 books. He read the first one in one week reading 25 pages everyday. He read the second book everyday and completed it in 12 days, it had 23 pages.

Either we are testing their understanding of grammar (in which case question can be made half confusing) or we are testing their mathematical ability (in which case we make the grammar as simply to understand as possible). Even my 'better wording' can be improved to make it simpler.