The way I think about it (using letters for math) is you are starting a warehouse and you need to find out how many (X) shelves of space to fit knowing only 4 items fit every EVERY time on a shelf. So you need to buy shelves to stack on and 36 items to stack, how many shelves am I going to need to have to fit everything?
You know: X shelves WITH 4 on a shelf OFFERS 36 spots
SO LOGICALLY: X shelves * 4 on a shelf = 36 spots
GETTING OUR SHELVES REORGANIZED (making sure the same on both sides)
LEADS TO: X shelves * 4 on a shelf / 4 on a shelf = 36 spots / 4 on a shelf
Reorganizing: X shelves = 36 spots / 4 on a shelf
CLEANING UP; X =36 / 4
Finally: X = 9 shelves
