1000 to 1999 = 1,000 numbers
Do the same for 3xxx, 5xxx, 7xxx, and 9xxx,
and you wind up with 5,000 numbers in all.
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Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
There are 12 such numbers.
9,000
There are 7200 such numbers.
500 or so.