Monthly sales of a school's co-op for a particular year are as follows:

January	February	March	April	May	June
45,300	30,300	10,500	9,500	9,000	10,400

July	August	September	October	November	December
25,200	20,000	20,300	27,500	15,300	50,400

a). Calculate the mean and standard deviation of the co-op's sales.

b). Estimate confidence interval for mean of sales at 95 per cent level. Show and explain your working and interpret your answer