Question 1132040
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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.     <U>ANSWER</U>




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 <U>answer</U>.
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Solved.