Given: CP of 18 pen = SP of 12 pen Formula used: Gain% = (Gain × 100)/CP Gain = SP – CP Calculation: Let SP of 1 pen be Re 1 So, SP of 18 pens = Rs. 18 CP of 18 pens = SP of 12 pens = Rs. 12 Gain = 18 – 12 = Rs. 6 Gain % = (6/12) × 100) = 50% ∴ The Gain% is 50%