The mileage (the distance a car can travel on one liter of gasoline) of a car is measured. It is known that the measurement technique has a standard deviation of 1 kilometer (the result is random due to the influence of air temperature and wind, tire types, etc.) and produces results that are normally distributed with zero mean error. Twenty five measurements were made and the average value of 12 km/l was obtained.

(a) Find the 90% and 95% confidence intervals for the mileage.

(b) What is the confidence level of the confidence interval [11.5, 12.5] km/l?

(c) How many measurements should be made to obtain the margin of error no more than 0.2 km/l at the 99% confidence level?