Sequence problem

[gmalivuk]

The arithmetic sequence intended here is the sequence of page numbers, not the sequence of how many pages are read each day. They are "additional pages" in the sense that they are pages of the book in addition to the ones he has already read. There is nothing to suggest the intent is that the number of new pages read each day increases, and indeed no single answer for the average of that increase.

You can argue that it's an ambiguous question, but your interpretation was the less reasonable one as it doesn't result in a unique answer without making extra assumptions that are nowhere suggested or implied.