The marketing manager of a large retail company wants to analyze the number of small appliances sold at their stores. She selects a random sample of 70 stores, and records the number of small appliances sold in the month of September. Data show mean sales of 53 units of small appliances with a standard deviation of 12 units. a)Compute a 90% confidence interval for the average number of units sold at the stores. [2 pts] b)A similar study conducted last year showed that the stores sold on average 50 units. The marketing manager is happy because sales are up from 50% to 53%, that is a 3% increase. Explain in a language that the manager can understand why she cannot be certain that sales increased from last year. [2 pts]