# Thread: Basic Probability Question Help

1. ## Basic Probability Question Help

I need help with 2 questions:

1) In a shipment of 18 trucks to a local truck dealer, there are 4 trucks that don't have air conditioning. Assume you select 4 trucks at random.

What is the probability that two of the four don't have air conditioning?
What is the probability that two or fewer of the four don't have air conditioning?

I tried to use permutation for this problem, but I'm not sure if that is right or which numbers to use: n!/ (n-x)! or maybe n! (n-r)r!

2) The volume of a one-pound bag of coffee is normally distributed. Suppose you take a random sample of 15 one-pound bags of coffee. Find two values K sub L and K sub U such that the probability is 95% that the ratio of the sample standard deviation divided by the population standard deviation is between K sub L and K sub U.

I don't understand what the question is asking or what K sub L and U are to be? Can both these questions be done on excel? If so, what formula?

Thanks

2. ## Re: Basic Probability Question Help

1. In selection, order doesn't matter.. so you can use combination formula.
check hyper geometric distribution for more help

2. hint: confidence interval. Find the distribution of sample standard deviation ( hint inside hint: chi square)

3. ## Re: Basic Probability Question Help

Originally Posted by vinux
1. In selection, order doesn't matter.. so you can use combination formula.
check hyper geometric distribution for more help

2. hint: confidence interval. Find the distribution of sample standard deviation ( hint inside hint: chi square)
thanks for the hints

2nd question is bonus so we haven't gone through that stuff yet :/

will post back after I tried these.

Ok so I got the answer for the first one using hyper geometric distribution function on excel.

For the 2nd question, I know that K sub L and U are two points on opposite sides of the mean. What I don't understand is the phrase "Find two values K sub L and K sub U such that the probability is 95% that the ratio of the sample standard deviation divided by the population standard deviation is between K sub L and K sub U." and how to use the standard dev divided by the pop std dev.

4. ## Re: Basic Probability Question Help

Will using the excel formula "=CHIINV(0.95,14)" give me the standard deviation of the sample or population? It gave me 6.57 but I'm not sure what that number is.

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