Understanding the Cost Price and Selling Price
To calculate the gain percent, we first need to determine the cost price (CP) and the selling price (SP) of the pens.
Cost Price Calculation
- Pens are bought at 12 for a rupee.
- Therefore, the cost price of one pen is:
- CP = 1 rupee / 12 pens = 1/12 rupee per pen.
Selling Price Calculation
- The selling price of one pen is given as 1 rupee.

Calculating Gain
- Gain can be calculated using the formula:
- Gain = Selling Price - Cost Price
- For one pen:
- Gain = 1 rupee - (1/12 rupee)
- Gain = 1 rupee - 0.0833 rupee
- Gain = 0.9167 rupee per pen.
Calculating Gain Percent
- Gain percent is calculated by the formula:
- Gain Percent = (Gain / Cost Price) * 100
- Substituting the values:
- Gain Percent = (0.9167 rupee / (1/12 rupee)) * 100
- Gain Percent = (0.9167 * 12) * 100
- Gain Percent = 11 * 100 = 1100%.

Conclusion
The gain percent on the pens is 1100%. This indicates a very high profit margin, reflecting effective pricing strategies in buying and selling.