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