To find the gain percentage, you can use the following formula:
Gain percentage = (Gain / Cost) * 100
In this case, the cost is 100 - 15 = Rs.85 and the gain is 15, so the gain percentage is:
Gain percentage = (15 / 85) * 100
This simplifies to:
Gain percentage = 15/17* 20
So the gain percentage is 300/17%.