.
It can be calculated and presented by different ways.
The simplest and most straightforward way is to notice that this set of numbers is EXACTLY the set of all 4-digit numbers from 1000 to 9999,
which consists of 9999 - 999 = 9000 numbers. ANSWER
Another way is to count them as follows:
Any of 9 digits from 1 to 9 in the most left position. 9 options
Any of 10 digits from 0 to 9 in the next position. 10 options.
---------------------- " ------------------------ 10 options
---------------------- " ------------------------ 10 options
In all, there are 9*10^3 = 9000 such numbers. The same answer.
Solved.