(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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

2. Relevant equations

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

3. 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?

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# Homework Help: Permutations combinatorics problem?

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