A Planning engineer is doing his medium term plans. Demand for the next 3 months is estimated to be 240, 480, 420 in terms of aggregate units. Shortages, backordering or sub-contraction is not allowed. The company have a regular and overtime capacity of 180 and 200 units per month respectively. The regular time and overtime a production cost are $100 and $200 per unit, respectively. Inventory cost is $20 per unit per month. Beginning inventory is zero, and ending inventory should be planned to be zero. Use Transportation Approach and solve the transportation problem by use of Northwest Corner Method.