If the two page numbers of an open book add to 137 what are the two page numbers?

When a book is open to pages 68 and 69, the sum of the page numbers is 137. If you wanted to solve a problem like this algebraically, you could call the left-handed page x, and the right-handed page x+1, since you know it's one larger than the left page. Then write x + (x + 1) = 137.

Subtract 1 from each side and you have x+x =136 or 2x = 136. Dividing both sides by 2, we have x = 68, so the left page is 68 and the right is 69.