1 The time spent by employees of a company surfing the web a

1.

The time spent by employees of a company surfing the web are normally distributed with a mean of 8.3 minutes and a standard deviation of 2.8 minutes. Find the probability that their time spent on the web:

a) Lasts between 7 and 12 minutes.

b) Lasts more than 9 minutes.

c) Lasts less than 6 minutes.

2.

The lifetimes of televisions that are advertised to last for 15,000 hours are   normally distributed with a mean of 15,100 hours and a standard deviation of 2000 hours. What is the probability that a television set lasts longer than the advertised figure?

3.

Travel-net is internet based travel agency where customers can view videos of the cities they plan to visit. The number of daily hits is a normally distributed random variable with a mean of 100,000 and a standard deviation of 24,000.

a) What is the probability of getting more than 120,000 hits?

b) What is the probability of getting fewer than 90,000 hits?

4.

The lifetimes of monitors produced by the Tishibi Company are normally distributed with a mean of 750 months and a standard deviation of 80 months. If the manufacturer wants to replace only 2 % of the monitors, what should the warranty be?

5.

The final marks in a statistics course are normally distributed with a mean of 75 and a standard deviation of 15. The professor has to convert all marks to letter grades. She decides that she wants 15 % of the grades to be A’s, 25 % of the grades to be B’s, 35% of the grades to be C’s, 20% of the grades to be D’s, and 5 % of the marks to be F’s. Determine the cutoffs for each letter grade. You want to find the lowest mark you can get for each letter grade.

Solution