Xiao Ming was reading a storybook. He had already read 40% of the book, and there were still 90 pages left. How many pages does this book have?

This book has a total of $90+ 40%$ $×$pages. Substituting 40% into the above formula: $90+04×$ $×$pages = $90+04×$ $×$ (100-90) = $90+04×$ $× $10 = $90+04×$ $× $100 = $90+40=130$pages Therefore, the book had a total of 130 pages.