ControlReturn on investment $($ROI$)$ is computed in the following manner: ROI is equal turnover multiplied by earnings as a percent of sales. Turnover is sales divided by total investment. Total investment is current assets $($inventories$,$ accounts receivable, andcash$)$ plus assets. Earnings equal sales minus the cost ofsales. The costof sales consists of variable production costs, selling expenses, freight anddelivery, and administrative costs.

a$.$ Construct an influence diagram that relates these variables.

b$.$ Define symbols and develop a mathematical model.