Question

How many $4$ digit number are there in all?

Answer 1

Estimate the total count of required natural numbers:

The smallest $4$ digit number is $1000$.

The largest $4$ digit number is $9999$.

Therefore, the total number of $4$-digit numbers

$=(9999-1000)+1$

$=8999+1$

$=9000$

Hence, there are $9000$ total $4$ digit numbers.