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?