Answer the question based on the information given. A machine produces pieces at the rate of 100 per hour. The cost production is Tk. 5 per unit. Each unit sells at Tk. 10. However, 10 per cent of the production is defective and all defective items are scrapped. If production is increased by x, then the cost of production increases by 0.5x per cent and (10 + 0.8x) per cent of the items are defective. The highest percentage increase in the production rate that keeps the profit positive is: