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Diagram 9, shows a four-digit passcode '0131' set by San on his smartphone.

Diagram 9

He wants to reset the passcode such that the new passcode cannot consists of digit 1 followed by digit 3. How many different passcodes can be formed? I need your help. Thanks.

my acc
  • 11
  • Always better to add your attempt. – user577215664 Jun 15 '20 at 15:54
  • As total is $10^4$ we count which cannot be formed and subtract. the forms are $13,,13,,13,,1313$ which gives $99+100+99+1=299$ codes can't be formed. Did I get the question right? Or $13,,1*3,,1*3$ are forbidden as well? Thanks. – Alexey Burdin Jun 15 '20 at 15:57
  • @AlexeyBurdin Thanks for your answer. So, different passcodes can be formed is $10^4-299=9701$ – my acc Jun 15 '20 at 16:38

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