We want to conduct a survey of those responsible for purchasing office products from service companies to obtain an average estimate of the annual amount of office supply purchases. An initial survey of service companies establishes the order of magnitude of the standard deviation of the annual amount of purchases at $800. I a) Determine the required number of service companies to estimate, with a confidence level of 90%, the average annual amount of office supply purchases, and this with a margin of error not exceeding $200. It is assumed that the annual purchase amount is distributed normally. b) For the same level of confidence, how many service companies must be contacted to ensure that the margin of error in the estimate of the average annual amount of office supply purchases does not exceed $100