Question

How many three - digit nubmers are there?

Answer 1

We cannot have a 0 at the hundred's place. So, the hundred's place can be filled with any of the 9 digits 1, 2,3....,9. So, there are 9 ways of filling the hundred's place.
Ten's place can be filled with any 10 digits in 10 ways. Now the unit's place can be filled with any 10 digits in 10 ways. Hence, the total number of required number = $9\times 10\times 10=900$