The sum of the page numbers on facing pages a book is 81 What are numbers?

Let x the page number on one side. The page number on the facing page is x+1. Their sum is:

$$x+(x+1)=81$$

Combining like terms:

$$2x+1=81$$

Subtracting 1 from both sides:

$$2x=80$$

Dividing both sides by 2:

$$x=40$$

So the page numbers are 40 and 41.