Question 395815
The numbers x, x+1, ..., y were taken that day. To find the number of people who visited, we find the size of the set {x, x+1, ...y}. 


Subtracting x-1 from all terms of the set this is equivalent to the set {1, 2, ..., y-x+1}. Therefore y-x+1 customers visited the service center.


Note that the answer is not simply y-x...this is because the set is inclusive.