SOLUTION: A telephone number consist of seven digits the first three representing the exchange. How many different telephone numbers are possible with in the 537 exchange?

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The last 4 digits determine how many numbers there are in the exchange.
0000
0001
0002
0003 ....
9996
9997
9998
9999
Because this involves INCLUSIVE numbers, we subtract the first number from the last and then ADD 1. The reason we add one is that we have in INCLUDE the fisrt one.


So you can see that this really works, lets look at a simpler problem:
You are supposed to read pages 1 - 5 in your history book. How many pages dod you read?
1
2
3
4
5
A total of 5