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perl

  • 3 years ago

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

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  1. KABRIC
    • 3 years ago
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    dont really get it

  2. UsukiDoll
    • 3 years ago
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    choose milk chocolates choose white chocolates and just 4 dark chocolates XD

  3. Chelsea04
    • 3 years ago
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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

  4. perl
    • 3 years ago
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    oh woops

  5. perl
    • 3 years ago
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    i thought it said 'any'

  6. Chelsea04
    • 3 years ago
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    do you get the question now?

  7. perl
    • 3 years ago
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    yes

  8. UsukiDoll
    • 3 years ago
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    I'm thinking milk white milk white milk white milk white milk white

  9. perl
    • 3 years ago
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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

  10. Chelsea04
    • 3 years ago
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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

  11. perl
    • 3 years ago
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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

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

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

  14. perl
    • 3 years ago
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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

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

  16. perl
    • 3 years ago
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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

  17. perl
    • 3 years ago
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    can i get a yes?

  18. Chelsea04
    • 3 years ago
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    sure, i guess

  19. perl
    • 3 years ago
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    lol

  20. Chelsea04
    • 3 years ago
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    the explanation seems reasonable enough

  21. perl
    • 3 years ago
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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?

  22. perl
    • 3 years ago
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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 )

  23. Chelsea04
    • 3 years ago
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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

  24. perl
    • 3 years ago
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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) + ...

  25. Chelsea04
    • 3 years ago
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    btw, what level of maths is this?

  26. perl
    • 3 years ago
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    not sure

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

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

  29. perl
    • 3 years ago
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    im in university

  30. Chelsea04
    • 3 years ago
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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...

  31. perl
    • 3 years ago
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    i find this problem confusing

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

  33. perl
    • 3 years ago
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    im trying to force these into mutually exclusive possibilities

  34. perl
    • 3 years ago
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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)

  35. perl
    • 3 years ago
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    but are these equally likely?

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

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

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

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

  40. perl
    • 3 years ago
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    what was your approach?

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

  42. perl
    • 3 years ago
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    can you show me your work

  43. perl
    • 3 years ago
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    i need your help :)

  44. perl
    • 3 years ago
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    |dw:1366635152002:dw|

  45. Chelsea04
    • 3 years ago
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    hmm..i guess you could use combinatorics

  46. Chelsea04
    • 3 years ago
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    does order matter?

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

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

  49. Chelsea04
    • 3 years ago
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    what do you get from that?

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

  51. perl
    • 3 years ago
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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

  52. perl
    • 3 years ago
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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 ?

  53. perl
    • 3 years ago
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    i could have used letters,

  54. Chelsea04
    • 3 years ago
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    sure

  55. perl
    • 3 years ago
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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)

  56. perl
    • 3 years ago
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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

  57. perl
    • 3 years ago
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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

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

  59. Chelsea04
    • 3 years ago
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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

  60. Chelsea04
    • 3 years ago
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    it's close to 4/5 at least

  61. perl
    • 3 years ago
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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

  62. KABRIC
    • 3 years ago
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    perl your answer is right thx

  63. perl
    • 3 years ago
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    and you could solve it using the complement approach .

  64. perl
    • 3 years ago
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    kabric, how do you know?

  65. Chelsea04
    • 3 years ago
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    yea, basically what i was thinking

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

  67. Chelsea04
    • 3 years ago
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    i just typed it?

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

  69. perl
    • 3 years ago
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    that is 8C8 , 16 C 12

  70. perl
    • 3 years ago
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    do you how it comes out the same ?

  71. Chelsea04
    • 3 years ago
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    what do you mean?

  72. Chelsea04
    • 3 years ago
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    you sort of lost me there

  73. perl
    • 3 years ago
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    oh ok sorry

  74. perl
    • 3 years ago
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    |dw:1366637463112:dw|

  75. Chelsea04
    • 3 years ago
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    yes...

  76. perl
    • 3 years ago
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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

  77. perl
    • 3 years ago
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    we cant have 9 dark ones, since there arent 9 dark ones

  78. Chelsea04
    • 3 years ago
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    sure

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

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

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