The selling price of a commodity can be calculated using the formula:

Selling Price = Cost Price + Profit

Given that the shopkeeper sold the commodity at Rs 500, we need to find the selling price at a profit of 20%.

Let's break down the problem into steps:

1. Determine the cost price of the commodity
Since the shopkeeper sold the commodity at Rs 500, the selling price is equal to the cost price plus the profit. Let's assume the cost price of the commodity is C.

500 = C + Profit

2. Calculate the profit percentage
The shopkeeper incurred a loss of 20% on the cost price. To calculate the profit percentage, we need to find the percentage of the cost price that will result in a profit of 20%.

Loss Percentage = 20%

Profit Percentage = 100% - Loss Percentage = 100% - 20% = 80%

3. Calculate the cost price
To find the cost price, we can use the formula:

Cost Price = Selling Price / (1 + (Profit Percentage / 100))

Substituting the values:

C = 500 / (1 + (80 / 100)) = 500 / (1 + 0.8) = 500 / 1.8 ≈ 277.78

Therefore, the cost price of the commodity is approximately Rs 277.78.

4. Calculate the selling price at a profit of 20%
To calculate the selling price at a profit of 20%, we can use the formula:

Selling Price = Cost Price + (Profit Percentage / 100) * Cost Price

Substituting the values:

Selling Price = 277.78 + (20 / 100) * 277.78 = 277.78 + 0.2 * 277.78 = 277.78 + 55.556 ≈ 333.33

Therefore, the shopkeeper must sell the commodity at approximately Rs 333.33 to make a profit of 20%.

In conclusion, the shopkeeper must sell the commodity at Rs 333.33 to make a profit of 20% after incurring a loss of 20%.