Answer the question based on the information given below:
Mr. David manufactures and sells a single product at a fixed price in a niche market. The selling price of each unit is Rs. 30. On the other hand, the cost, in rupees, of producing ‘x’ units is 240 + bx + cx2, where ‘b’ and ‘c’ are some constants. Mr. David noticed that doubling the daily production from 20 to 40 units increases the daily production cost by 66(2/3)%. However, an increase in daily production from 40 to 60 units results in an increase of only 50% in the daily production cost. Assume that demand is unlimited and that Mr. David can sell as much as he can produce. His objective is to maximize the profit.
Question: How many units should Mr. David produce daily?