Thornton Manufacturing Company set its standard variable manufacturing cost at $28 per unit of product. The company planned to make and sell 3,900 units of product during Year 3 . More specifically, the master budget called for total variable manufacturing cost to be $109,200. Actual production during Year 3 was 4,100 units, and actual variable manufacturing costs amounted to $115,630. The production supervisor was asked to explain the variance between budgeted and actual cost ($115,630$109,200= $6,430 ). The supervisor responded that she was not responsible for the variance that was caused solely by the increase in sales volume controlled by the marketing department. Required a. Determine the flexible budget variance and indicate the effect of the variance by selecting favorable (F) or unfavorable (U). Note: Select "None" if there is no effect (i.e., zero variance). b. Do you agree with the production supervisor