Problem 2. On a shelf, there are three books. The first has 90, the second 110, and the third 150 pages. The covers of the books are of equal thickness, and each of them is 2 mm thick. How many millimeters thick are the books together if it is known that 10 pages have a thickness of $1 \mathrm{~mm}$?
Solution. Each book has two covers, so the total number of covers is six, and their total thickness is $12 \mathrm{~mm}$. The total number of pages is

$$
90+110+150=350
$$

So the total thickness of all the pages is $350: 10=35 \mathrm{~mm}$. Finally, the total thickness of all three books together is $12+35=47 \mathrm{~mm}$.