Explanation: Assume Market Price (MP) of 1 pen is 100. Cost Price (CP) for 40 pens = MP of 36 pens = \(36 \times 100 = 3600\). Selling Price (SP) of 1 pen with \(1\%\) discount on MP = \(100 \times (1 - 0.01) = 99\). SP for 40 pens = \(40 \times 99 = 3960\). Profit = \(SP - CP = 3960 - 3600 = 360\). Percentage of profit = \((Profit / CP) \times 100\% = (360 / 3600) \times 100\% = 10\%\).