Hypothesis testing for mean with t-test

a) A manufacturer of a plasticised line used in home-assembly mobiles advertises that their product has an average tensile strength of 30 kilograms (this is a measure of how strong the product is). You took a sample of 144 sections of the line and tested them. The average tensile strength of this sample was 28 kilos, with a standard deviation of 12 kilos. Does this enable you to dismiss the manufacturer's claims?

b) The EPA reports that the exhaust emissions for a certain car model has a normal distribution with a mean of 1.45 grams of nitrous oxide per mile. The car manufacturer claims their new process reduces the mean level of exhaust emitted for this car model. A random sample of 30 cars is taken and the mean level of exhaust emitted for this sample is 1.21 grams with a standard deviation of 0.4.

c) A credit card company wondered whether giving frequent flyer miles for every purchase would increase card usage, which has a current mean of $2500 per year. They gave free miles to a random sample of 60 credit card customers and found the sample mean to be $2542.  The sample standard deviation is $109.

d) Studies conducted in the 1970s indicated that the average age at which children take their first alcoholic drink is 14.6 years old. Sociologists believe that children are starting to drink at a younger age. A random sample of 169 young adults (18 years of age) is selected and the age at which each adult took their first alcoholic drink is recorded. The sample mean age was 13.3 years of age with a standard deviation of 5 years.