7. (10 points) Lei Ke bought a new book and liked it very much. On the first day, he read $\frac{1}{5}$ of the book plus 12 pages. On the second day, he read $\frac{1}{4}$ of the remaining pages plus 15 pages. On the third day, he read $\frac{1}{3}$ of the remaining pages plus 18 pages. At this point, there were still 42 pages left to read. So, the total number of pages in the book is $\qquad$ .
【Analysis】By using the reverse method, the conclusion can be drawn.
【Solution】Solution: The remaining pages on the second day are $(42+18) \div \frac{2}{3}=90$ (pages), the remaining pages on the first day are $(90+15) \div \frac{3}{4}=140$ (pages), and the total number of pages in the book is $(140+12) \div \frac{4}{5}=190$ (pages). Therefore, the answer is 190.