Question 240496
In general, the formula is: last - first + 1.<br>
To understand this, let's look at the example you gave: How many numbers from 100 to 1000?<ul><li>First, how many numbers are there from 1 to 1000? Clearly (I hope) there are 1000 numbers.</li><li>Of the numbers from 1 to 1000, which ones are <b>not</b> included in the numbers from 100 to 1000? Again I hope it is clear that 1 through 99 are not included.</li><li>The number of numbers from 100 to 1000 would be the number of numbers of 1 to 1000 minus the number of numbers from 1 to 99: 1000 - 99 = 901.</li></ul>
Let's look at a general case. Let L = the last (or highest number) and F = the first (or lowest number in the sequence. That makes (F-1) the highest number that is not in the sequence.<br>
Therefore the  number of numbers in the sequence will be: the number of numbers from 1 to L (which is L) minus the number of numbers from 1 to (F-1) (which is (F-1):
L - (F-1) = L - F + 1
which is the formula I gave you at the beginning.<br>
Here's an example: How many numbers are there from 536 to 2349? Answer: 2349 - 536 + 1 = 1814<br>
I hope this helps.