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9
9 ways
one digit: 5 ways two digits: 5 * 9 ways three digits: 5 * 9 * 8 ways four digits: 5 * 9 * 8 * 7 ways ... ten digits: 5 * 9! ways So if you have to use ten digits, then the answer is 5 * 9! = 1814400. If you can use anywhere from one to ten digits, then the answer is: 5 * (9! + 8! + 7! + 6! + 5! + 4! + 3! + 2! + 1) = 2045565
There are 9*8*7 = 504 ways.
There are 9!/2! = 181,440 ways.