Problem 3b. What key assumption did you make in order to be able to use the formula shown in Problem 3a above? Explain [2 marks] Problem 3c Referring to Variable 3, what sample size is required assuming a maximum confidence interval width of 10% of the sample mean and a 95% level of confidence? To answer this, assume that the population standard deviation equals the sample standard deviation (just for Problem 3c). You need to show the appropriate formula with the right numbers substituted in the right places in this formula and you need to use your calculator to compute the final answer. [4 marks]