The correct answer is 7.
Here's how I got my answer:
In order to find the "number", we would need to find the difference between the two numbers. We can do this by doing 
 , which is your answer, 7.
, which is your answer, 7.
Example: To find the difference of 20 and 40, we would need to do 
 , which is 20.
, which is 20.