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A six digits code chosen out of 0-9 digits. ‘003425’ for example. How many codes are there with at least two times the digit ‘7’?

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  • $\begingroup$ The number of codes without "7" is? The number of codes with exactly one "7" is? $\endgroup$ – peterwhy Nov 9 '14 at 13:34
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Hint:

  • how many codes are there?
  • how many codes are there without digit $7$?
  • how many codes are there with digit $7$ appearing exactly once?
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