Android Lock Screen

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The Android Finger-Lock screen consists of 9 dots on a square. A code on it consists of 4 to 9 dots visited in a specific order. Assume you can always go from one dot to any non-visited dot. (In reality some motions are not possible, but you can ignore that for this problem.)



How many codes are possible on the Android lock screen?

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  • I don't understand this one:
    it shouldn't be 9!, because the first time you can choose among 9, but the second input is a choise among 8, because you can't reselect the one you are on
    should't it be
    (9*8*8*8)+(9*8*8*8*8)+(9*8*8*8*8*8)+(9*8*8*8*8*8*8)+(9*8*8*8*8*8*8*8)+(9*8*8*8*8*8*8*8*8) ?

  • @Jacopo, 9! would give you the number of 9-dot permutations, but you also want the total number of 4,5,6,7, and 8 dot permutations. After picking 2 dots, you no longer have 8 dots left...

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