According to a recent survey, Americans are keeping their cars longer. Some of the reasons may include the poor economy and better-built vehicles. A random sample of new- and used-car owners was asked how long they usually kept their car (in months).
a. Find a 99% confidence interval for the true mean time people keep a new car.
b. Find a 99% confidence interval for the true mean time people keep a used car.
c. Using your answers in parts (a) and (b), do you think new-car owners and used-car owners keep their cars for different lengths of times? Justify your answer.
Along with gold and silver, platinum is also considered a precious metal and is traded on the commodities market. A random sample of the price of platinum (in dollars per troy ounce) was obtained from jewelers around the world. The resulting summary statistics were
Assume the underlying distribution of the price is normal.
a. Find a 95% confidence interval for the current true mean price per troy ounce of platinum.
b. One month ago, a brokerage firm reported the true mean price of platinum to be $660.79 per troy ounce, with a recommendation to buy. Is there any evidence to suggest the true mean price has increased? Justify your answer.