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## Statistics and probability

### Course: Statistics and probability > Unit 7

Lesson 1: Basic theoretical probability# 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.

## Want to join the conversation?

- What would be the percentage of the question in the video?(27 votes)
- The answer in a percentage will be 37.5%(24 votes)

- what the dog doin?(10 votes)
- we don't know yet(2 votes)

- how do u do probability when it says =

for example p(3 or multiple of 2)?(8 votes) - What is the probability of me getting my mommys credit card and spending her entire bank balance on vbucks?!! Also, what is the probability of me getting my dad's credit card and spending all of his money on shiny rocks in gorrilla tag?!(8 votes)
- 0% in getting any and if you manage to there is a 100% chance you will get...(4 votes)

- Do we use rounding for probability?(6 votes)
- 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)

- Hey there! This comment will serve as a chat, a place where you can vent, or rant! Have fun! ^^(5 votes)
- POV ur bored and your mom is forcing you to watch this for videogames. Who voted?(5 votes)
- 4352 Orlando. St. Toronto Canada.(2 votes)
- 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)
- one two buckle my shoe three four shut the door five six pick up sticks(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.