Let the total number of pages in this book be x pages. Reading 20% of the book on the first day meant reading 20% * x pages, which was 02x pages. The next day, he read 25% of the total number of pages, which meant that he read 25% * x pages, which was 025x pages. There were 110 pages left. According to the above calculation, the equation can be listed: 02x + 025x + 110 = x Solve this equation: 3x = 110 x = 33 Therefore, the total number of pages in this book was 33.