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Question
How many 9-digit numbers of different digits can be formed?
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Solution
Since the first digit cannot be zero, number of ways of filling the first digit = 9
Number of ways of filling the second digit = 9 (as repetition is not allowed or the digits are distinct)
Number of ways of filling the third digit = 8
Number of ways of filling the fourth digit = 7
Number of ways of filling the fifth digit = 6
Number of ways of filling the sixth digit = 5
Number of ways of filling the seventh digit = 4
Number of ways of filling the eighth digit = 3
Number of ways of filling the ninth digit = 2
Total such 9-digit numbers = `9xx9xx8xx7xx6xx5xx4xx3xx2=9(9!)`
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