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bahrom7893

  • 2 years ago

Four students place their phones on a desk. Later they each pick up a phone at random. What is the probability that exactly one student gets his/her phone?

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  1. bahrom7893
    • 2 years ago
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    @satellite73 can u help me out.

  2. Arnab09
    • 2 years ago
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    1/4

  3. bahrom7893
    • 2 years ago
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    Arnab that's wrong. The probability that the first student gets his phone is 1/4th. What if the 2nd guy gets the phone. Then one phone is gone.

  4. bahrom7893
    • 2 years ago
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    So the probability that second guy gets it is 1/3

  5. bahrom7893
    • 2 years ago
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    ash, the answer's 1/3

  6. bahrom7893
    • 2 years ago
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    idk how they got it though. Exactly one student gets the phone back.

  7. waqassaddique
    • 2 years ago
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    100 % probability if they are in senses. lol but it ll be 1/24

  8. Arnab09
    • 2 years ago
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    oh sorry, lemme think..

  9. lizaing
    • 2 years ago
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    can someone please help me? x :)

  10. Arnab09
    • 2 years ago
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    yeah, it is coming 1/3 :)

  11. bahrom7893
    • 2 years ago
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    I'm thinking: (1/4) "first student got" + (2nd|Nobody else got theirs)

  12. bahrom7893
    • 2 years ago
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    and then 3rd got his given nobody else got theirs and then 4th got his, etc..

  13. Arnab09
    • 2 years ago
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    first, pick up a phone at random and give it to the right person. it can be done in 4C1 ways= 4 ways. rest of the 3 are to be deranged, so, D3= 2 ways. so, the condition can be satisfied in total 4*2 ways=8 ways out of 4! ways= 24 ways. so, the probability is 8/24=1/3

  14. Arnab09
    • 2 years ago
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    got it, @ bahrom7893?

  15. bahrom7893
    • 2 years ago
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    what do u mean by deranged? this may be a dumb question, but I've never heard of the term before.

  16. Arnab09
    • 2 years ago
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    u know about derangement?

  17. bahrom7893
    • 2 years ago
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    nope.

  18. Arnab09
    • 2 years ago
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    ok, derangement is a kind of arrangement where no right thing goes to right place..

  19. bahrom7893
    • 2 years ago
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    Ohh ok. What is the formula for derangement?

  20. Arnab09
    • 2 years ago
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    http://en.wikipedia.org/wiki/Derangement

  21. Arnab09
    • 2 years ago
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    there is a general method: Dr= r!(1/2!-1/3!+1/4!-..... upto 1/n!)

  22. Directrix
    • 2 years ago
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    This problem is an updated version of the Secretary's Packet Problem A secretary types four letters to four people and addresses the four envelopes. If she inserts the letters at random, each in a different envelope, what is the probability that exactly three letters will go into the right envelope? http://www.cut-the-knot.org/Probability/IntuitiveProbability.shtml Generalized here: http://www.jstor.org/discover/10.2307/2690041?uid=3739616&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=55851236913

  23. Arnab09
    • 2 years ago
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    sorry, last term is 1/r! ^^

  24. bahrom7893
    • 2 years ago
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    kk ty everyone!

  25. Arnab09
    • 2 years ago
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    welcome :)

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