584 Views 3 Replies Latest reply: Aug 14, 2002 12:00 AM by TKHunny
4 posts since
Aug 14, 2002
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Hi, I would like to make sure I have done these problems correctly. Please show me how to do these 6 problems.....

1. Using the appropriate distribution answer the following questions.
Number of times = ,0,1,2,3,4 and 5. Proportion = 0.1, 0.2, 0.1, 0.4, and 0.2.
Question is, The number of connections on the Internet during any two minutes period is given by the following distribution:
A. Determine the mean number of times a connections is made during a two minute period?
B. Determine the standard deviation of the number of connections made during a two minute period?

2. According to a recond survey, 70 percent of all customers will return to the same grocery store. Supose eight customers are selected at radom, what is the probability that:
A. Exactly five of the customers will return?
B. All eight will return?
C. At least seven will return?
D. At least one will reurn?
E. How many customers will be expected to the same store?

3. In a statistic class with 15 males and 13 females, five students are selected to put problems on the board. what is the probabilty that:
A. 3 females and 2 males are selected?
B. All 5 students selected are males?
C. All 5 students selected are females?
D. At least one male is selected?

4. Fix-it Copiers advertises a mean time of 100 minutes for office calls with a standard deviation of 25 minutes. What percentage of calls are completed:
A. Between 100 and 120 minutes?
B. In less than 120 minutes?
C. In less than 60 minutes?
D. Between 120 and 150 minutes?
E. Between 60 and 120 minutes?
F. 20 percent of their jobs take more than how much time?

5. Access the time to a supercomputer mainframe is normally distributed with an average (mean) of 92 minutes a day with a standard deviation of 11 minutes. Find the probability that on a certain day, the access time is:
A. less than 60 minutes?
B. More than 100 minutes?
C. Between 70 and 90 minutes?
D. between 1 and 2 hours?
E. On 15 percent of the days, the access time is more than how many minutes?

6. A file cabinet manufacturer estimates that 5 percent of its file cabinets will have drawers that close improperly. Assume a production run of 40 cabinets is completed:
A. What is the mean and standard deviation of this distribution?
B. What is the probability that 3 or more are defective?
C. what is the probability that 3 cabinets are defective?
D. What is the probability that there are more than 4 defective file cabinets?
Lois

• 1,835 posts since
Nov 10, 1999
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Aug 14, 2002 12:00 AM (in response to ljdoyle1-disabled)
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We cannot do your homework for you. If you tell us what you have tried or what you came up with, we can make sure you are right or point you the right direction.

Craig
• 2,520 posts since
Sep 25, 2009
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Aug 14, 2002 12:00 AM (in response to ljdoyle1-disabled)
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#1 is ill-defined. You have provided 6 possible outcomes but only 5 probabilities.

Anyway, Mean = E[x] = sum[x�pr(x)]
Variance = E[x2] - Mean2

It is a matter of multiplying and adding up.

#2 is a binomial distribution with p = 0.7 and n = 8

You can then construct the desired probabilities.

pr(at least 1) = 1 - pr(zero)

In this case, with n = 8, you can easily construct the entire distribution and essentially just read off the answers. Of course, as the sample size increases, constructing the entire distribution becomes tiresome rather quickly.

You show us your answers and some indication of how you arrived at them, then we can be of more use to you.

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