Why do you add +1 in counting test questions? Here's an example question from the SAT question of the day:

On the last day of a one-week sale, customers numbered 149 through 201 were waited on. How many customers were waited on that day?
  Possible answers: 51, 52, 53, 152, 153.

The correct answer here is 53, which is the result of (201-149) + 1 = 53.
What's the reasoning in adding the +1?

Always take in mind that subtracting number $b$ from $a$ means that how much should be added to $b$ to reach a?
Therefore, there is an obvious different between subtraction and counting. Your problem is about counting from 149 to 201, so that you must include 149 in your counting (but subtraction exclude 149 [see the italic sentence at the first line!] and it is the reason to add +1). Subtraction means that How many numbers (or distance -Real nubmers case) are between two numbers.
