The area of a rectangular deck is 680 square ft. the deck's width is 17 ft. what is its length?

Area formula is length times width. You are given the total area and the width, so you only need to find the length. What your have been given can be set up as an equation:
17 ft times length ft (x) = 680 ft^2
x = 680/17 ft
x = 40 ft.

Hope that helps:)

Well if you have 20 bats you probably want 20 balls? you don’t have enough context to answer the question.