How many ten-digit numbers have at least two equal digits?
Please explain!

Between 1,000,000,000 and 9,999,999,999 there are 9,000,000,000 different ten-digit numbers. Of those, 9*9! (9 times 9 factorial) = 3,265,920 have all ten digits different, i.e., no two equal digits. Take the difference of those two numbers, and you will have your answer.

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