Hey there!
We know that each book contains 20 stamps. We're looking for how many 20's we can have to get 300.
Remember, multiplication also means "repeated addition".
We're looking for how many 20's we can add to get 300.
We'll multiply 20 by the number of books we'll need, b, we'll solve for b.
We have:
20b = 300.
We divide both sides by 20, to get 15 books.
You must buy 15 books to fit 300 stamps.
I hope this helped you!