MHB Probability that the total annual profit is not more than 800000

  • Thread starter Thread starter mathmari
  • Start date Start date
  • Tags Tags
    Probability
AI Thread Summary
The discussion focuses on calculating the probability that the total annual profit of an insurance office, with 2,500 contracts, does not exceed $800,000. The mean annual profit per contract is $330, and the standard deviation is $540. Participants confirm that the total profit follows a normal distribution with parameters $N(2500 \mu, 2500 \sigma^2)$. They discuss the necessity of applying the central limit theorem, concluding it is unnecessary since the sum of independent normal variables is also normal. The conversation emphasizes the correct formulation of the probability expression and the importance of notation in calculations.
mathmari
Gold Member
MHB
Messages
4,984
Reaction score
7
Hey!

An insurance office has $2500$ contracts with mean annual profit (per contract) $\mu=330$ and standard deviation $\sigma=540$. Calculate the probability that the total annual profit is not more than $800000$.

I have one the following:

The annual total profit should be not more than $800000$, that means that per contract it shoulebe not more than $\frac{800000}{2500}=320$.

So $$Z=\frac{X-\mu}{\sigma}=\frac{320-330}{540}\approx -1.31 \\ P(Z\leq -1.31)=0.0968$$ Is that the correct probability?
 
Physics news on Phys.org
Hey mathmari!

Let $n=2500$ be the number of contracts,
Let $X_i$ be the annual profit of contract $i$.
Let $X=\sum\limits_{i=1}^{n} X_i$ the annual profit.

So we want to know $P(X\not >800k)=P(X\le 800k)$.

We assume that the $X_i$ are independent and have distribution $N(\mu, \sigma^2)$.
Can we tell what the distribution of $X$ is? 🤔
 
Klaas van Aarsen said:
Let $n=2500$ be the number of contracts,
Let $X_i$ be the annual profit of contract $i$.
Let $X=\sum\limits_{i=1}^{n} X_i$ the annual profit.

So we want to know $P(X\not >800k)=P(X\le 800k)$.

We assume that the $X_i$ are independent and have distribution $N(\mu, \sigma^2)$.
Can we tell what the distribution of $X$ is? 🤔

Isn't $X$ also normal with $N(2500 \mu, 2500 \sigma^2)$ ?
 
mathmari said:
Isn't $X$ also normal with $N(2500 \mu, 2500 \sigma^2)$ ?
Yep. (Nod)

Can we calculate $P(X\le 800k)$ now?
I think it comes out slightly different than what you wrote before. 🤔
 
Klaas van Aarsen said:
Yep. (Nod)

Can we calculate $P(X\le 800k)$ now?
I think it comes out slightly different than what you wrote before. 🤔

It is $$P\left (\frac{\sum_{i=1}^{2500}X_i-2500\mu}{2500\sigma}\leq 8000000\right )$$ or not?
 
We have to apply central limit theorem, or not? :unsure:
 
mathmari said:
We have to apply central limit theorem, or not?

No need.
We already know that the sum of independent random variables with identical normal distributions is again a normal distribution.
So we don't need the $n=2500$ to be sufficiently large. 🧐

mathmari said:
It is $$P\left (\frac{\sum_{i=1}^{2500}X_i-2500\mu}{2500\sigma}\leq 8000000\right )$$ or not?

As you wrote before, the variance is $2500\sigma^2$.
So the standard deviation cannot be $2500\sigma$, can it? (Shake)

Btw, we need parentheses in the numerator.
Otherwise we have $\sum_{i=1}^{2500}X_i-2500\mu = (\sum_{i=1}^{2500}X_i)-2500\mu \ne \sum_{i=1}^{2500}(X_i-2500\mu)$. 🧐
 
Last edited:
Back
Top