One-litre cartons of milk are advertised to contain 1 l of milk. To test this claim, the Consumer Council of South Africa measured a random selection of 20 cartons from supermarket shelves. They found that the average fill was 0.982 litres, with a sample standard deviation of 0.068 l. Assume that carton fills are approximately normally distributed.
(a) Formulate the null and alternative hypothesis to test the claim that one-litre cartons of milk are being underfilled. Conduct a hypothesis test for a single mean using α = 0.05.
(b) Use Excel's TDIST function to find the p-value for this test. Interpret the findings for the Consumer Council of South Africa.