The difference between 91 and 21 is: 
The difference between 52 and 18 is: 
The difference between 93 and 21 is: 
The difference between 91 and 20 is: 
The difference between 51 and 21 is: 
The difference between 91 and 71 is: 
The difference between 90 and 21 is: 
The difference between 91 and 21 is: 