Return on investment ( ROI) is computed in the fol-lowing manner: ROI is equal to turnover multiplied by earnings as a percent of sales. Turnover is sales divided by total investment. Total investment is cur-rent assets ( inventories, accounts receivable, and cash) plus fixed assets. Earnings equal sales minus the cost of sales. The cost of sales consists of variable production costs, selling expenses, freight and delivery, and administrative costs. a. Construct an influence diagram that relates these variables. b. Define symbols and develop a mathematical model.