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9000 of them. All of the numbers 1 thru 9999 EXCEPT 1 thru 999. 9999 - 999 = 9000.
9000 of them. All of the numbers 1 thru 9999 EXCEPT 1 thru 999. 9999 - 999 = 9000.
The sum of all numbers between 1 and n = n*(n+1)/2 The sum of all numbers between 1 and 999 = 999*1000/2 = 499500 The sum of all numbers between 1 and 9999 = 9999*10000/2 = 49995000 So, the sum of all numbers between 1000 and 9999 = 49495500
9999
Infinitely many. There are, in fact, infinitely many numbers between any two numbers.