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perl

A box contains 8 dark chocolates, 8 white chocolates, and 8 milk chocolates. I choose chocolates at random (yes, without replacement; I’m eating them). What is the chance that I have chosen 20 chocolates and still haven’t got all the dark ones? what is your answer to that

  • one year ago
  • one year ago

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  1. KABRIC
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    dont really get it

    • one year ago
  2. UsukiDoll
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    choose milk chocolates choose white chocolates and just 4 dark chocolates XD

    • one year ago
  3. Chelsea04
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    the question says that you still haven't got ALL the dark ones, so you COULD get 4 dark chocolates to 7 dark chocolates, but not the full 8

    • one year ago
  4. perl
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    oh woops

    • one year ago
  5. perl
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    i thought it said 'any'

    • one year ago
  6. Chelsea04
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    do you get the question now?

    • one year ago
  7. perl
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    yes

    • one year ago
  8. UsukiDoll
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    I'm thinking milk white milk white milk white milk white milk white

    • one year ago
  9. perl
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    you wantthe probability of not getting all 8 dark chocolates. you are guaranteed at least 4 dark chocolates so the possibilities are 4 dark, 5 dark, 6 dark, 7 dark

    • one year ago
  10. Chelsea04
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    yep, so if you find the probability of getting 8 dark chocolates, you can minus it from 1 and you'll get the probability of getting 4 dark, ... 7 dark

    • one year ago
  11. perl
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    there are 4 favorable cases, out of 5 total possible cases favorable cases : : exactly 4 dark , 16 non-dark : exactly 5 dark 15 non-dark exactly 6 dark 14 non-dark exactly 7 dark 13 non-dark total cases : exactly 4 dark , 16 non-dark : exactly 5 dark 15 non-dark exactly 6 dark 14 non-dark exactly 7 dark 13 non-dark exactly 8 dark , 12 non-dark I see no reason not to treat these choices as equally likely. therefore the probability of not picking all dark ones is 4/5

    • one year ago
  12. perl
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    since they must add up to 20 , these are the only possibilites

    • one year ago
  13. perl
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    notice 3 dark 17 non-dark is impossible

    • one year ago
  14. perl
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    the situation is constrained by two facts there must be no more than 16 non-dark chocolates there must be at least 4 dark chocalates

    • one year ago
  15. perl
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    that produces (16,4) (15, 5 ) (14,6) (13, 7) (12,8)

    • one year ago
  16. perl
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    the situation is constrained by two facts you cannot have more than 16 non-dark chocolates you cannot have more than 8 dark chocolates there must be at least 4 dark chocalates so the possibilities are (# non-dark, # dark) (16,4) (15, 5 ) (14,6) (13, 7) (12,8) there is 4/5 favorable

    • one year ago
  17. perl
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    can i get a yes?

    • one year ago
  18. Chelsea04
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    sure, i guess

    • one year ago
  19. perl
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    lol

    • one year ago
  20. Chelsea04
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    the explanation seems reasonable enough

    • one year ago
  21. perl
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    i think its more complicated, let X = number of dark ones , Y = number of non-dark chocolate how many ways can you choose 16 non dark and 4 dark? how many ways can you choose 15 non-dark and 5 dark?

    • one year ago
  22. perl
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    P( chance not getting all dark in 20 draws) = P( X = 4 & Y= 16 or X= 5 & Y=15 or X=6 & Y=14 or X=7 or Y=13 )

    • one year ago
  23. Chelsea04
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    that's what i thought too, but if you add up all the ways to not get all 8 dark chocolate, i'm like 90% sure you'll get 4/5...cause there are like 60 ways if you account for each individual selection

    • one year ago
  24. perl
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    P( X = 4 & Y= 16 or X= 5 & Y=15 or X=6 & Y=14 or X=7 or Y=13 ) = P ( X=4 & Y=16) + P(X=5 & Y=15) + ...

    • one year ago
  25. Chelsea04
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    btw, what level of maths is this?

    • one year ago
  26. perl
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    not sure

    • one year ago
  27. Chelsea04
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    i meant like are you still in school or university?

    • one year ago
  28. perl
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    P(Y=16 & X=4) = P(Y=16) * P (X=4 | Y = 16) =

    • one year ago
  29. perl
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    im in university

    • one year ago
  30. Chelsea04
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    oh, then you might not want to listen to me, i'm in yr 11 doing yr 12 maths so yea...

    • one year ago
  31. perl
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    i find this problem confusing

    • one year ago
  32. Chelsea04
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    whoops, should have told you that at the start!

    • one year ago
  33. perl
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    im trying to force these into mutually exclusive possibilities

    • one year ago
  34. perl
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    you cannot have more than 16 non-dark chocolates you cannot have more than 8 dark chocolates there must be at least 4 dark chocalates so the possibilities are (# non-dark, # dark) (16,4) (15, 5 ) (14,6) (13, 7) (12,8)

    • one year ago
  35. perl
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    but are these equally likely?

    • one year ago
  36. perl
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    if these possibilities are equally likely to occur (and they are mutually exclusive), then the probability is 4/5

    • one year ago
  37. Chelsea04
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    i'd assume so, unless some were heavier or bigger than the others (which isn't so)

    • one year ago
  38. perl
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    im use to working on problems like 5/12*4/11 , those sorts of problems

    • one year ago
  39. Chelsea04
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    me too, I guess you could do that, but you'd be there forever!

    • one year ago
  40. perl
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    what was your approach?

    • one year ago
  41. Chelsea04
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    I found the probability of getting all 8 chocolates, kinda took a while and subtracted it from 1

    • one year ago
  42. perl
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    can you show me your work

    • one year ago
  43. perl
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    i need your help :)

    • one year ago
  44. perl
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    |dw:1366635152002:dw|

    • one year ago
  45. Chelsea04
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    hmm..i guess you could use combinatorics

    • one year ago
  46. Chelsea04
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    does order matter?

    • one year ago
  47. perl
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    because i want Probability ( 4 dark& 16 non-dark) exactly

    • one year ago
  48. perl
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    \[\frac{ 8C4*16C16 }{24C20 }+\frac{ 8C5*16C15 }{24C20}+\frac{ 8C6*16C14}{24C20}+\frac{ 8C7*16C13 }{24C20 }\]

    • one year ago
  49. Chelsea04
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    what do you get from that?

    • one year ago
  50. perl
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    the first is the case 4 dark & 16 non-dark then 5 dark& 15 nondark

    • one year ago
  51. perl
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    the order of eating them does not count, but we should label in our mind the chocolates. if you want, label the dark chocolates 1-8, and label the non-dark chocolates 9-24

    • one year ago
  52. perl
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    so now we have the problem dark chocolate = { 1,2,3,4,5,6,7,8} , non-dark { 9,10,11,12,... 23,24} , the numbers are going to be labels for the chocolates, ok ?

    • one year ago
  53. perl
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    i could have used letters,

    • one year ago
  54. Chelsea04
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    sure

    • one year ago
  55. perl
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    so for instance, suppose we want four dark chocolates, and 16 non-dark chocolate. so we do probability = # favorable outcomes / # total possibilities the favorable, first choose 4 dark chocolates which has a total of 8 choose 4 ways to do it, then multiply by that how many ways can you choose 16 non-dark from 16 non dark. there is only one way. so im using multiplication rule (and probability)

    • one year ago
  56. perl
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    the denominator is the total number of ways to choose 20 chocolates , and there are 24 choose 20 ways to choose 20 chocolates

    • one year ago
  57. perl
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    anyways, show me your work, and we can compare answers . sorry if my explanation is confusing, im not exactly sure of the correct jargon

    • one year ago
  58. perl
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    so I got a probability of 629/759 = .8287

    • one year ago
  59. Chelsea04
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    I tried using permutations so like: \[\frac{ nPr(16,16)*nPr(8,4)+nPr(16,15)*nPr(8,5)...etc }{ nPr(24,20) }\] but then the amswer was: \[\frac{ 5 }{ 245157 }\] which doesn't seem right...AT ALL... so i replaced all the P with C to use combinatorics and i got 629/759, which seems more accurate than permutations

    • one year ago
  60. Chelsea04
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    it's close to 4/5 at least

    • one year ago
  61. perl
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    thats right, and yes its close to 4/5, but 4/5 would give you a wrong answer. i know how teachers are, lol

    • one year ago
  62. KABRIC
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    perl your answer is right thx

    • one year ago
  63. perl
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    and you could solve it using the complement approach .

    • one year ago
  64. perl
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    kabric, how do you know?

    • one year ago
  65. Chelsea04
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    yea, basically what i was thinking

    • one year ago
  66. perl
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    chelsea, how did you get nCr using pretty print?

    • one year ago
  67. Chelsea04
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    i just typed it?

    • one year ago
  68. perl
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    \[1-\frac{ \left(\begin{matrix}8 \\ 8\end{matrix}\right)*\left(\begin{matrix}16 \\ 12\end{matrix}\right) }{ \left(\begin{matrix}24 \\ 20\end{matrix}\right) }\]

    • one year ago
  69. perl
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    that is 8C8 , 16 C 12

    • one year ago
  70. perl
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    do you how it comes out the same ?

    • one year ago
  71. Chelsea04
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    what do you mean?

    • one year ago
  72. Chelsea04
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    you sort of lost me there

    • one year ago
  73. perl
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    oh ok sorry

    • one year ago
  74. perl
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    |dw:1366637463112:dw|

    • one year ago
  75. Chelsea04
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    yes...

    • one year ago
  76. perl
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    P( X>7 ) = P(X=8 or X=9 or .. ) but X=9 and higher is impossible so we only need to look at X = 8

    • one year ago
  77. perl
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    we cant have 9 dark ones, since there arent 9 dark ones

    • one year ago
  78. Chelsea04
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    sure

    • one year ago
  79. perl
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    ok lets not pursue this further, i have a new question answering, follow me

    • one year ago
  80. WalterDe
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    Correct answer => http://629a3486.linkbucks.com

    • 11 months ago
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