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General multiplication rule example: dependent events

AP.STATS:
VAR‑4.D (LO)
,
VAR‑4.D.2 (EK)
We can use the general multiplication rule to find the probability that two events both occur when the events are not independent. Created by Sal Khan.

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  • stelly blue style avatar for user Jalena Crawford
    What is the difference between mutually exclusive event and the independent event?
    (2 votes)
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    • leaf green style avatar for user kubleeka
      Two events are independent if the occurrence of either event doesn't affect the probability of the other. Coin tosses are independent because a coin has no memory of previous flips; each toss has 50% chance of heads, no matter the previous results.

      Two events are mutually exclusive if only one of them can occur. If I toss a coin, the events 'heads' and 'tails' are mutually exclusive, because they cannot both occur on the same toss.
      (5 votes)
  • leaf green style avatar for user 石乐志大师
    I have another question. What is the possibility that Doug doesn't draw silk?
    (1 vote)
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    • piceratops ultimate style avatar for user ANB
      The key to this question is finding the probability that Doug draws silk.

      In order for Doug to get silk, Maya first has to not get silk (5/6 chance) then Doug has to draw silk (1/5 chance).

      So this means that Doug has a (5/6)*(1/5) or 1/6 chance of drawing silk. If he has a 1/6 chance of drawing silk, then that means that he has a 5/6 chance of not drawing silk.
      (1 vote)
  • blobby green style avatar for user AlexanderD
    The words that was in the sentence.
    (0 votes)
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Video transcript

- [Instructor] We're told that Maya and Doug are finalists in a crafting competition. For the final round, each of them will randomly select a card without replacement that will reveal what the star material must be in their craft. Here are the available cards. So I guess the star material is the primary material they need to use in this competition. Maya and Doug both want to get silk as their star material. Maya will draw first, followed by Doug. What is the probability that neither contestant draws silk? Pause this video and see if you can work through that before we work through this together. All right, now let's work through this together. So the probably that neither contestant draws silk. So that would be, I'll just write it another way, the probability that, I'll write MNS for Maya no silk. So Maya no silk and Doug no silk. That's just another way of saying, what is the probability that neither contestant draws silk? And so this is going to be equivalent to the probability that Maya does not get silk, Maya no silk, right over here, times the probability that Doug doesn't get silk, given that Maya did not get silk. Given Maya no silk. This line right over, this vertical line, this is shorthand for given. And so let's calculate each of these. So this is going to be equal to the probability that Maya gets no silk. She picked first there's six options out of here. Five of them are not silk, so it is five over six. And then the probability that Doug does not get silk, given that Maya did not get silk. So Maya did not get silk, then that means that silk is still in the mix, but there's only five possibilities left because Maya picked one of them, and four of them are not silk. There's still silk as an option. And it's important to recognize that the probability that Doug gets no silk is dependent on whether Maya got silk or not. So it's very important to have this given right over here. If these were independent events, if Maya picked and then put her card back in and then Doug were to pick separately, then the probability that Doug gets no silk, given that Maya got no silk, would be the same thing, as a probability that Doug gets no silk regardless of what Maya was doing. And so this will end up becoming four over six which is the same thing as two thirds.