Question

How many four-digit numbers are there in all?

Answer 1

There are 10 digits i.e., 0, 1, 2, 3, 4, 5, 6 ,7, 8, 9.
We can not use '0' at thousand's place.
So, we can use only 9 digits at thousand's place.
Also, we can use 10 digits at hundrend's, 10 digits at ten's and 10 digits at unit's place.
So, total numbers of four-digit numbers = 9 × 10 × 10 × 10 = 9000