The number of 5 digits telephone numbers having at least one of their digit repeated is

A-90000 b-10000 c-30240 d-69760
Kindly give me explained answer because i am in learning stage.

First assuming all digits are possible there are 10^5 different 5 digit numbers.
Now calculate how many of these have no repetitions. This is the number of
permutations of 5 digits drawn from 10 =10P5 =10!/5!=30240.
The final answer is 10^5 - 30240=d