Just the other day, one of my students was having trouble remembering how to find the unit price of an item when given the price and weight of an item. This student of mine was always confused with whether it is the price divided by the weight, or the weight divided by the price.
An example is as follows:
What is the unit price (per gram) of a bag of rice that weighs 500g and is selling for $9?
As I was explaining to this student, it suddenly dawned on me that we are finding $/g. When we write this down, we can clearly see that we should use the price divided by the weight (Price/weight = $/g = $9/500g). My student got it immediately. I learned something new as well... :)