20. A company produces three kinds of light bulbs: a 60-watt soft-lite bulb, a 60-watt regular bulb, and a 100-watt bulb. Each bulb takes one hour per case in production line 1. In production line 2, a case of the soft-lite takes 2 hours and a case of each of the others takes 1 hour. Production line 1 has 25 hours available per week and line 2 has 40 hours per week. The company has determined that the two 60-watt bulbs are considered substitute products by most people and that their combined demand will not be more than 25 cases per week. The demand for 100-watt bulbs will never be greater than 60 cases per week. If the soft-lite bulbs earn a profit of Rs 70 per case and other two bulbs earn a profit of Rs 50 per case, how many cases per week should the company produce of each type in order to maximise profits?

Solve the above problem, write its dual and provide its economic interpretation. From the optimal solution tableau, obtain the optimal values of the dual variables.