Unit 4 Lesson 3 Assignment

1) Quality control for Speedie Typewriters, Inc., has done studies showing that the light-use model has a mean life of 3.5 years and a standard deviation of 0.7 years. At the prescribed usage, how long should the guarentee period be if the company wishes to replace no more than 10% during the guarentee period?

2) The life of a Freeze Breeze electric fan is normally distributed with a mean of 4 years and standard deviation 1.2 years. The manufacturer will replace any defective fan free of charge while it is under guarentee. For how many years should a Freeze Breeze fan be gaurenteed if the manufacturer does not want to replace more than 5% of them?

3) The final exam scores in a statistics class were normally distributed with a mean of 63 and a standard deviation of 5.

a) What score has 10% of the values above it?

b) What score has 70% of the values below it?

4) More and more households in the United States have at least one computer. The computer is used for office work at home, research, communication, personal finances, education, entertainment, and a myriad of other things. Suppose the average number of hours a household personal computer is used for entertainment is 2 hours per day. Assume the times for entertainment are normally distributed and the standard deviation for the times is half an hour.

a) How many hours of computer use for entertainment would be in the bottom 25%?

b) How many hours of computer use for entertainment would be in the top 15%?

5) At two years of age, sardines inhabiting Japanese waters have a length distribution that is approximately normal with mean 20.20 cm and standard deviation 0.65 cm.

a) How long are the longest 15% of all these sardines?

b) How short are the shortest 15% of all these sardines?

6) The mean cholesterol level in children is 175 mg/dL with standard deviation 35 mg/dL. Assume this level varies from child to child according to an approximate normal distribution. How high are the levels for the highest 2% of all children?

7) Porphyrin is a pigment in blood protoplasm and other body fluids that is significant in body energy and storage. In healthy Alaskan brown bears, the amount of porphyrin in the bloodstream (in mg/dL) has approximate normal distribution with mean 47.5 and standard deviation 12.2. How low are the porphyrin levels for the lowest 3% of all the bears

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