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### Course: Statistics and probability > Unit 9

Lesson 7: Geometric random variables- Geometric random variables introduction
- Binomial vs. geometric random variables
- Probability for a geometric random variable
- Geometric probability
- Cumulative geometric probability (greater than a value)
- Cumulative geometric probability (less than a value)
- TI-84 geometpdf and geometcdf functions
- Cumulative geometric probability
- Proof of expected value of geometric random variable

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# Cumulative geometric probability (less than a value)

Probability for a geometric random variable being less than a certain value.

## Want to join the conversation?

- Why we are not considering P(C = 0)?(10 votes)
- Think about it. P(C = 0) is actually 0, since C = 0 means 0 cake order, right? Without a cake order, there is 0% probability of receiving an order over the telephone. You could include it, if you want to, but it just evaluates to 0, so we don't include it.(24 votes)

- Is that right?

The way I understand is: what's the probability that she receives**at least**1 telephone order in the 1st 4 orders. The complement of which is**no**telephone orders in the 1st 4 orders, so 1 - p(no telephone order)(5 votes)- She's receiving orders
*until*she gets the first telephone one. So it is not about**at least 1**.(3 votes)

- Hi, it seems to me that here we are calculating the probability that C = 4. Is that really the same as calculating the probability that C<5?

If our condition is C<5, then C could take on 1,2,3 or 4, all in equal probabilities I assume. Are we taking that into account?(3 votes)- actually I think I got it. Because we are actually adding up all probabilities lower than C=5. So we are adding P(C=1) + P(C=2) + P(C=3) + P(C=4). So we are taking all those probabilities into account.(5 votes)

- @Khanacademy could you write the formulas on the lesson page? I am trying to make revision notes and it's a bit annoying having to watch the videos each time to find the formula :)(4 votes)
- Once again, the use of "until" causes unnecessary confusion. To P(C=1) is the (.9)*(.1). C is the number of cake orders she receives UNTIL she gets a telephone order. That means she will already have a regular cake order (which has a ".9" probability) before receiving the telephone order (which has a ".1" probability). The P(C=0), however, would be .1. If Sal wants clarity, so that we're properly tested on a our knowledge of how to apply the formula, it would be better to use "before" instead of "until".(3 votes)
- We are asked to find the probability that it takes less than 5 orders for Lilyana to get her first telephone order.

This obviously includes the telephone order, and thereby it makes sense to also let 𝐶 include the telephone order, i.e. 𝐶 ∈ {1, 2, 3, 4, ...}, and from Sal's explanation of his work it shouldn't come as a surprise that this is indeed the way he chose to define 𝐶.(2 votes)

## Video transcript

- [Instructor] Lilyana runs
a cake decorating business, for which 10% of her orders
come over the telephone. Let C be the number of cake
orders Lilyana receives in a month until she first gets
an order over the telephone. Assume the method of placing
each cake order is independent. So C, if we assume a few things, is a classic geometric random variable. What tells us that? Well, a giveaway is that we're gonna keep doing these independent trials, where the probability
of success is constant, and there's a clear success. A telephone order in
this case is a success. The probability is 10% of it happening. And we're gonna keep doing
it until we get a success. So classic geometric random variable. Now they ask us, find the
probability, the probability, that it takes fewer than five orders for Lilyana to get her first
telephone order of the month. So it's really the probability
that C is less than five. So like always, pause this
video and have a go at it. And even if you struggle
with it, that's better. Your brain will be more
primed for the actual solution that we can go through together. Alright. (chuckles) So I'm assuming you've had a go at it. So there's a couple of
ways to approach it. You could say, well, look,
this is just gonna be the probability that C is equal to one, plus the probability
that C is equal to two, plus the probability
that C is equal to three, plus the probability
that C is equal to four. and we can calculate it this way. What is the probability that C equals one? Well, it's the probability
that her very first order is a telephone order. And so we'll have 0.1. What's the probability that C equals two? Well, it's the probability
that her first order is not a telephone order. So it's one minus 10%. There's a 90% chance it's
not a telephone order, and that her second order
is a telephone order. What about the probability C equals three? Well, her first two orders
would not be telephone orders, and her third order would be one. And then C equals four? Well, her first three orders
would not be telephone orders, and her fourth one would. And we could get a calculator maybe, and add all of these things up, and we would actually get the answer. But you're probably wondering, well, this is kind of hairy
to type into a calculator. Maybe there is an easier
way to tackle this. And indeed, there is. So think about it. The probability that C is less than five, that's the same thing as
one minus the probability that we don't have a telephone
order in the first four. One minus the probability
that no telephone order in first four orders. So what's this? Well, 'cause this is just saying, what's the probability we do have an order in the first four? So it's the same thing as
one minus the probability that we don't have an
order in the first four. And this is pretty
straightforward to calculate. So this is going to be equal to one minus, and let me do this in another color so we know what I'm referring to. So what's the probability that
we have no telephone orders in the first four orders? Well, the probability on a given order that you don't have a
telephone order is 0.9. And then if that has to be
true for the first four, well, it's gonna be 0.9
times 0.9 times 09 times 0.9, or 0.9 to the fourth power. So this a lot easier to
calculate, so let's do that. Let's get a calculator out. Alright, so let me just
take .9 to the fourth power is equal to, and then let
me subtract that from one. So let me make that a negative, and then let me add one to it. And we get, there you go, 0.3439. So this is equal to 0.3439. And we're done. That's the probability that it
takes fewer than five orders for her to get her first
telephone order of the month.