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Answer:
You see, a six digit
has one million possible combinations instead of 10,000.
Step-by-step explanation:
The sequences you describe are exactly the numbers from 0 (which is the same thing as 000000) to 999999. There are one million of them (999999 + 1).
If repetition is not allowed (which is probably what you are referring to in your last sentence), you can pick any of ten digits for the first number, any of the nine remaining for the second, and so forth. This is 10 times 9 times 8 times 7 times 6 times 5 or 151,200.
A quicker way of writing this is: 10! / 4! (where n! = n times n-1 times n-2, all the way down to one) -- or more generally n! / (n-r)!, where n is the number of possible digits (zero to nine) and r is the number you get to pick (six in this case).