Unit is related to Class. Class is related to Instructor. 
Hard is related to Coin. you are likely to find Coin in/at/on Purse. you are likely to find Cash in/at/on Purse. 
Unit <--RelatedTo--> Class <--RelatedTo--> Instructor 
Hard <--RelatedTo--> Coin --AtLocation--> Purse <--AtLocation-- Cash 