Tim wants to collect 400 stickers. if he makes pages of 10, how will he know when he has 400 stickers?
If he puts 40 stickers on each page (4 by 10) then he will know 40 stickers make up 1 page out of 10. Since there are 10 pages, you'd multiply by how many stickers per page. 40 times 10 is 400.