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Summer Clouds moderators

07/11/2017 at 11:16
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If a room has 7 doors, in how many ways can a person enter through one door and exit through a different door?




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    Dao Trong Luan Coordinator 07/11/2017 at 11:56

    If this person start go:

    - At first door, he/she can exit at 6 rest doors 

    - At second door, he/she can exit at 5 rest doors 

    - At third door, he/she can exit at 4 rest doors 

    - At fourth door, he/she can exit at 3 rest doors 

    - At fifth door, he/she can exit at 2 rest doors 

    - At sixth door, he/she can exit at 1 rest doors 

    - And at last door, he/she can exit at 0 rest doors 

    So this person will have:

    6 + 5 + 4 + 3 + 2 + 1 + 0 = 21 [ways]

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