Question

How many numbers are there between 100 and 1000 in which all the digits are distinct?

Answer 1

648

This range gives all 3 digit nos.

_ _ _

1st place can be filled by 9 digits- 1–9 excluding zero as 0 on first place would not form a 3 digit no.

2nd place can be filled by 9 digits- 0–9 not including the 1st no.

3rd place can be filled by 8 digits- 0–9 not including he 1st 2 nos.

Therefore answer is 9*9*8= 64