Answer:
 inches 
Explanation:
I think your question is missed of key information, allow me to add in and hope it will fit the original one. 
Please have a look at the attached photo. 
My answer: 
Given the information in the photo, we know that:
- the length of string she used most is 
  - the length of string she used least often 
  
=> the difference in length between the length of string she used most often and the length of string she used least often is: 
= 
 - 
 
= 
 inches 
Hope it will find you well