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Simple probability: yellow marble

In order to find the probability of picking a yellow marble from a bag, we have to first determine the number of possible outcomes, and then how many of them meet our constraints. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • aqualine ultimate style avatar for user Tais Price
    What would be the percentage of the question in the video?
    (0 votes)
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  • aqualine ultimate style avatar for user starsky2004
    how do u do probability when it says =
    for example p(3 or multiple of 2)?
    (8 votes)
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  • old spice man blue style avatar for user gregoryp
    what the dog doin?
    (7 votes)
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  • starky tree style avatar for user dominic.eckman
    Do we use rounding for probability?
    (6 votes)
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    • leaf blue style avatar for user jwinder47
      Whether we round off our answers (or our intermediate calculations) depends on the problem we're trying the solve, and the conditions we are given. For the kinds of probability calculations we are doing here (that rely mostly on counting procedures) we don't round off, as a general rule.
      (0 votes)
  • leaf green style avatar for user acealena
    I had this question on a test and couldn't figure out the answer. Maybe one of you can help me. There are marbles in a box. Two-thirds of them are white, half of them are red, and 6 of them are green. How many marbles are in the box and what is the probability of selecting a white marble?
    (3 votes)
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  • marcimus pink style avatar for user Bella Mcphail
    What happens if you get a decimal?
    (2 votes)
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    • mr pink green style avatar for user David Severin
      Probability can be expressed in many ways, a ratio, a decimal, a fraction, or a percent. Since the maximum probability is 1 (or 100%), all probabilities could be expressed as a decima.
      Decimals may require rounding. As long as the decimal is less than 1, it is one of the ways to express probability.
      (2 votes)
  • starky sapling style avatar for user Trin
    I understand this but I need help with this question from DeltaMath "In a popular online role playing game, players can create detailed designs for their characters "costumes" or appearance. Shaniece set up a website where players can buy and sell these costumes online. Information about the number of people who visited the website and number of costumes purchased in a single day is listed below.
    97 visitors purchased no costumes
    26 visitors purchased exactly one costumes
    13 visitors purchased more then one costume
    Based on these results, express the probability that the next person will purchase more then one costumes as a percent to the nearest whole number "
    If you can please help.😁
    (2 votes)
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    • duskpin ultimate style avatar for user Krystal
      Well, I'm not great at this stuff, but I can at least try.
      There are 136 visitors, because 97 + 26 + 13 = 136. 13 visitors purchased more than one costume, so the probability is 13/136. I personally don't understand the last part, so I can't help you there.

      Hope this helped! :)
      Art3mis
      (2 votes)
  • male robot hal style avatar for user ravenshortcut
    3/4 thats the answer
    (2 votes)
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  • orange juice squid orange style avatar for user Apoidea
    What is the difference between basic probability, simple probability and discrete probability?
    (2 votes)
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  • male robot hal style avatar for user Greg L
    What if you want to know the probability of getting a yellow marble if you picked two marbles?
    (1 vote)
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    • purple pi pink style avatar for user ZeroFK
      Let's try to work this out. There are 8 marbles total, 3 of which are yellow. First you pick 1 marble, which has a chance of 3/8 to be yellow. If it's a yellow, great, you're done. But if it's not yellow, you get another chance. There are now 7 marbles left, and still 3 of them are yellow. So now the chance of getting a yellow on the second try is 3/7.
      Hmm so what would be the combined chance of both of those events - taking into account that the second event only needs to take place if the first event was a "negative" - i.e. the marble was not yellow. That's kinda hard to think about.

      It turns out my last sentence holds a clue to the solution: the second event only happens if the first was a negative. So what if we try to invert the problem: if you pick 2 marbles, what's the probability of not getting at least 1 yellow? That's easier: the first time it's 5/8, the second time it's 4/7, and the combination is just both those events happening. No need for "the second event only if the first didn't happen" or complicated things like that. The new question is simply: what is the chance of getting no yellows in 2 tries?

      I'll leave the details of the calculation to you - the result is 20/56 = 5/14.

      So what does this tell us? The chance of not getting a yellow after 2 tries is 5/14. But we wanted to know the inverse of that: the chance of getting at least 1 yellow. We get that by subtracting 5/14 from 1. After all, either we get a yellow or we don't: the sum of those two options must equal 100% = 1.

      So the probability of getting at least 1 yellow is:
      1 - 5/14 = 9/14 or about 64%.
      (3 votes)

Video transcript

Find the probability of pulling a yellow marble from a bag with 3 yellow, 2 red, 2 green, and 1 blue-- I'm assuming-- marbles. So they say the probability-- I'll just say p for probability. The probability of picking a yellow marble. And so this is sometimes the event in question, right over here, is picking the yellow marble. I'll even write down the word "picking." And when you say probability, it's really just a way of measuring the likelihood that something is going to happen. And the way we're going to think about it is how many of the outcomes from this trial, from this picking a marble out of a bag, how many meet our constraints, satisfy this event? And how many possible outcomes are there? So let me write the possible outcomes right over here, so possible outcomes. And you'll see it's actually a very straightforward idea. But I'll just make sure that we understand all the words that people might say, so the set of all the possible outcomes. Well, there's three yellow marbles. So I could pick that yellow marble, that yellow marble, or that yellow marble, that yellow marble. These are clearly all yellow. There's two red marbles in the bag. So I could pick that red marble or that red marble. There's two green marbles in the bag. So I could pick that green marble or that green marble. And then there's one blue marble in the bag. There's one blue marble. So this is all the possible outcomes. And sometimes this is referred to as the sample space, the set of all the possible outcomes. Fancy word for just a simple idea, that the sample space, when I pick something out of the bag, and that picking out of the bag is called a trial, there's 8 possible things I can do. So when I think about the probability of picking a yellow marble, I want to think about, well, what are all of the possibilities? Well, there's 8 possibilities, 8 possibilities for my trial. So the number of outcomes, number of possible outcomes, you could view it as the size of the sample space, number of possible outcomes, And it's as simple as saying, look, I have 8 marbles. And then you say, well, how many of those marbles meet my constraint, that satisfy this event here? Well, there's 3 marbles that satisfy my event. There's 3 outcomes that will allow this event to occur, I guess is one way to say it. So there's 3 right over here, so number that satisfy the event or the constraint right over here. So it's very simple ideas. Many times the words make them more complicated than they need to. If I say, what's the probability of picking a yellow marble? Well, how many different types of marbles can I pick? Well, there's 8 different marbles I could pick. And then how many of them are yellow? Well, there's 3 of them that are actually yellow.