Given:
- Cost price (CP) of the chair = ₹450
- Selling price (SP) of the chair = ₹500

To Find:
- Gain percentage

Formula:
- Gain percentage = (Profit / CP) × 100

Calculations:
The profit is the difference between the selling price and the cost price of the chair.
- Profit = SP - CP = ₹500 - ₹450 = ₹50

Now, we can substitute the values in the formula to calculate the gain percentage.
- Gain percentage = (₹50 / ₹450) × 100

Result:
The gain percentage is calculated to be 11.11%.

Explanation:
To find the gain percentage, we need to first calculate the profit. Profit is the difference between the selling price and the cost price. In this case, the selling price is ₹500 and the cost price is ₹450. So, the profit is ₹50.

To calculate the gain percentage, we use the formula: (Profit / CP) × 100. Substituting the values, we get (₹50 / ₹450) × 100 = 11.11%.

Therefore, the gain percentage for selling the chair for ₹500 after buying it for ₹450 is 11.11%.