Return on investment (ROI) is profit divided by investment. In marketing, ROI is determined as incremental sales times gross margin minus marketing investment, all divided by marketing investment. Suppose that a company plans to spend $3 million to place search engine ads and expects $15 million in incremental sales. Its gross margin is estimated to be 45%. 

a. Develop a spreadsheet to compute the marketing ROI. 

b. Use the spreadsheet to predict how ROI will change if the incremental sales estimate is wrong (consider a range of values above and below the expected sales).