A company produces a certain product and has the capacity to produce 100 units per month in regular time, and an additional 15 units per month in overtime. The production cost of the product varies by month due to changes in the material costs over time. The quantities of the product to be delivered over the next six months have already been set. The delivery requirements and production costs are given in the following table:

Production cost per unit in regular time rand Production cost per unit in overtime rand