The CP of a pen is Rs 80. It is sold at Rs 100. Find the gain per cent.

Answer

To find the gain percent when the cost price (CP) of a pen is Rs 80 and it is sold at Rs 100, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Cost Price (CP) and Selling Price (SP)**:
- Cost Price (CP) = Rs 80
- Selling Price (SP) = Rs 100

2. **Calculate the Profit**:
- Profit is calculated using the formula:
\[ \text{Profit} = \text{Selling Price} - \text{Cost Price} \]
- Substitute the values:
\[ \text{Profit} = 100 - 80 = 20 \text{ Rs} \]

3. **Calculate the Gain Percent**:
- Gain percent (or profit percent) is calculated using the formula:
\[ \text{Gain Percent} = \left( \frac{\text{Profit}}{\text{Cost Price}} \right) \times 100 \]
- Substitute the values:
\[ \text{Gain Percent} = \left( \frac{20}{80} \right) \times 100 \]

4. **Simplify the Calculation**:
- First, simplify \(\frac{20}{80}\):
\[ \frac{20}{80} = \frac{1}{4} \]
- Now multiply by 100:
\[ \text{Gain Percent} = \frac{1}{4} \times 100 = 25\% \]

5. **Conclusion**:
- The gain percent is 25%.