Total number of way to choose 4 digits number:
__ __ __ __
> first digit can be any number from 1 - 9. Please note, you can not include 0 because left most 0 digit will make the 3 digits number. For ex- 0123, 0456, 0444 etc.
> Second digit could be anything from 0-9.
> thirst digit could be anything from 0-9.
> fourth digit could be anything from 0-9.
total number of ways : 9 * 10 * 10 * 10
Total number of numbers with all 4 digits different:
> First digit could be anything from 1-9.
> One digit has been chosen, second digit could be anything from 1-9 ( minus 1 digit) or 0.
> 2 digits have been chosen, third digit could be anything from 0-9 ( minus 2 digits)
> 4th digit could be anything from 0-9 ( minus 3 digits).
Total number of way: 9 * 9 * 8 * 7
Now you can find an answer.
