TITLE: Find the number of $X$ from start to end
QUESTION [15 upvotes]: If I wanted to figure out for example, how many tutorial exercises I completed today.
And the first question I do is question $45$, 
And the last question I do is question $55$
If I do $55-45$ I get $10$.
But I have actually done $11$ questions:
$1=45$, $2=46$, $3=47$, $4=48$, $5=49$, $6=50$, $7=51$, $8=52$, $9=53$, $10=54$, $11=55$.
Is there any way to know when I can just subtract. Or is the rule I always have to add $1$ when I subtract?

REPLY [2 votes]: If you started at the question 15 and finished at the question 15; how many question have you answered?
Imagine a list of exercises to be done in order. Some of them are marked as done already. You start from the first unmarked question.

Every time you complete an excercise you mark it, thus increasing the number of the first excercise waiting to be done.

This way the number of excercises done in some time is an incremet of a number of the first excercise not done yet.
Since you started with question 45 (so 45 was the first question NOT done then) and you stopped after question 55 (so the first question NOT done yet is $56 = 55+1$ now), you have answered $(55+1)-45=11$ questions.
Put it another way:
when you started your work, the last question answered was number $44 = 45-1$, and now it is $55$; the number of questions you answered today is an increment of the number of questions answered: $55 - (45-1) = 11$.