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## Homework Statement

A telephone extension has four digits, how many different extensions are there with no repeated digits, if the first digit cannot be zero?

## Homework Equations

P(n,r)=n!/(n-r)!

## The Attempt at a Solution

For the first digit, there are 9 possibilities (because no zero)

For the last 3 digits I used P(9,3)=9!/6!=9x8x7

So, in the end my result was: 9x9x8x7= 4,536 different extensions...

I'm just wondering if I was correct?