Q. How many 7 digit telephone numbers can be formed using the numerals 0 to 9?

• 0 to 9 are total 10 numbers so we can fill 7 places with all 10 numbers so total no of telephone nos will be 10^7.
  but this will also contain telephone no as 0000000 so subtract 1 from total numbers hence
  total no of telephone nos will be (10^7)-1
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• 9*10^6..
  ...
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• The last six digits can be anything from 0 to 9, so we can place those 10 numbers in 6 positions with repetition, so the no. of ways =
  10*10*10*10*10*10 = 10^6
  First digit should not be 0, so only 9 digits for 1 position, the no. of ways = 9
  So the answer is 9*(10^6)
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• if digits are not repeated then 9c7*7!=181440
  if digits are repeated then 10^7
  
  because in the phone number can start with 0
  
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