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小 發表於 2017-3-14 01:50 PM (第 2616 天)
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Number of passwords can be formed with 3 letters not placing next to each other (and 4 numbers)
= 26P3 × 10P4 × 5C3
= 786240000
Number of passwords can be formed with 2 letters not placing next to each other (and 5 numbers)
= 26C2 × 10C5 × (7! - 2 × 6!)
= 294840000
Number of passwords can be formed with 1 letter (and 6 numbers)
= 26C1 × 10C6 × 7!
= 27518400
Number of passwords can be formed with no letters (7 numbers)
= 10P7 = 604800
The required number of passwords
= 786240000 + 294840000 + 27518400 + 604800
= 1109203200
[ 本帖最後由 loppy 於 2017-3-14 01:59 PM 編輯 ]
