# Thread: Stats question on probability

1. ## No questions...

Suppose a salesperson makes a sale on 25 percent of customer contacts. In a normal work week, the salesperson contacts 30 customers.

a. What is the probability that the salesperson will make no sales?

b. What is the probability that the salesperson will make at least two sales?

c. What is the probability that the salesperson will make two sales at most?

2. This is classic binomial probability. I don't have time to baby step, but here are the formulas. If you're as lost as you claim and don't know what the notation means, then you should possibly see your instructor.

The general formula is $\displaystyle C(n,k)p^{k}(1-p)^{n-k}$

a. $\displaystyle C(30,0)(.25)^{0}(.75)^{30}$

b. $\displaystyle 1-\left[\sum_{k=0}^{1}C(30,k)(.25)^{k}(.75)^{30-k}\right]$ or $\displaystyle \sum_{k=2}^{30}C(30,k)(.25)^{k}(.75)^{30-k}$

c. $\displaystyle \sum_{k=0}^{2}C(30,k)(.25)^{k}(.75)^{30-k}$

3. Originally Posted by Faritova
Suppose a salesperson makes a sale on 25 percent of customer contacts. In a normal work week, the salesperson contacts 30 customers.

a. What is the probability that the salesperson will make no sales?

b. What is the probability that the salesperson will make at least two sales?

c. What is the probability that the salesperson will make two sales at most?
(a) The person will make no sales only if all his attempts end in failure. The probability of failure is 1 - .25 = .75, thus the probability that we will make no sales is $\displaystyle (.75)^{30}$.

(b) Let $\displaystyle p$ be the probability of at least two sales. Then $\displaystyle 1-p$ is the probability of 0 sales or 1 sale. The probability of 0 sales is (a) which is $\displaystyle (.75)^{30}$ and the probability of 1 sale is $\displaystyle _{30}C_1 (.25)^1 (.75)^{30-1}$. You should be able to find from there.

4. Originally Posted by Faritova
First of all, hello to everyone here. I am thrilled there is a v bulletin that helps out for math.

I have a lot questions all relating to probablility. And this is going to be challenging for people to answer because I am math illiterate, I really need someone to explain the forumulas to me in a very very very slowed down method. Do not automatically assume I'm a math whiz or know the appropriate formula some kind soul is referring to. There is a very good chance I won't! I need to figure out how to do the following question.

Suppose a salesperson makes a sale on 25 percent of customer contacts. In a normal work week, the salesperson contacts 30 customers.

a. What is the probability that the salesperson will make no sales?

b. What is the probability that the salesperson will make at least two sales?

c. What is the probability that the salesperson will make two sales at most?

Now what I need to know is what formulas are being used. What are they called, how is the data slotted into that formula so that if I see questions like this I know what to do. Right now I am completely lost! Of course I can jokingly say that for question A his chances are not that great! He's make one sale in 25% of customer contacts, so 25% of 30 contacts, he's got a 1 in 7.5 chance, or .13 that he will make a sale and an .87 that he won't. How close is that to the truth? To tell the honest truth I have no idea what the heck I'm doing! HELP! And if the first part is even remotely correct, how the heck do you calculate the rest and like I said, if there's a formula, what's it name and how does it work in the most non math literate way you know how to explain! Walk me thorugh each and every step and be kind, I'm an English major stuck in a rut!