Consider the question. Jonas is a book collector. He has so many books he is converting his third bedroom into a library. This room has 400 square feet of space. He plans to put several bookshelves in the room and each bookshelf takes up 80 square feet of space. If he reserves 160 square feet of space to use for a desk and some walking space, how many shelves can he put in the room?
 If the answer is '3'; explain the reasoning:
If he reserves 160 sq. ft. of space for a desk and some walking space, then he will have 400 - 160 = 240 sq. ft. of space remaining. If each bookshelf takes up 80 sq. ft. of space, then 240 sq ft can contain 240 / 80 = 3 bookshelves.