1. The length of a screw produced by a machine is normally distributed with a mean of 0.65 inches and a standard deviation of 0.01 inches. What percent of screws are between 0.61 and 0.69 inches?
2. The probability that a train leaves on time is 0.8. The probability that the train arrives on time and leaves on time is 0.24. What is the probability that the train arrives on time, given that it leaves on time?
3. A study of one thousand teens found that the number of hours they spend on social networking sites each week is normally distributed with a mean of 20 hours. The population standard deviation is 2 hours. What is the 95% confidence interval for the mean?
4. A study of two hundred teens found that the number of hours they spend on social networking sites each week is normally distributed with a mean of 12 hours. The population standard deviation is 4 hours. What is the margin of error for a 98% confidence interval?