Answer:11.11percent gain

Verified answer

Sure, here is the solution to the problem:

• Let the selling price of one pen be Rs. x.
• Then, 18x = 20CP
• CP = 9x/10
• Profit/Loss % = (SP - CP)/CP * 100
• = (x - 9x/10)/9x/10 * 100
• = 100/9 = 11.11% profit.

Hope it helps...✨