To solve this problem:

Step 1: Cost Price (C.P.) per pen

The person buys 20 pens for Rs. 60. Therefore, the cost price per pen is: $C.P. = \frac{60}{20} = 3 \, \text{Rs.}$

Step 2: Selling Price (S.P.) per pen

The person sells 20 pens for Rs. 50. Hence, the selling price per pen is: $S.P. = \frac{50}{20} = 2.5 \, \text{Rs.}$

Step 3: Loss per pen

The loss per pen is: $\text{Loss} = C.P. - S.P. = 3 - 2.5 = 0.5 \, \text{Rs.}$

Step 4: Loss percentage

The loss percentage is calculated as: $\text{Loss \%} = \left( \frac{\text{Loss}}{\text{C.P.}} \right) \times 100 = \left( \frac{0.5}{3} \right) \times 100 = 16.67 \%$

Final Answer:

The loss percentage is 16.67%.

Let me know if you'd like a detailed breakdown of any step or have further questions!
Here are some related questions for practice:

1. What would be the profit percentage if the pens were sold at Rs. 70 for 20 pens?
2. Calculate the loss percentage if the pens were sold at Rs. 40 for 20 pens.
3. What is the break-even selling price per pen?
4. How does the loss percentage change if 25 pens were sold for Rs. 60?
5. If the cost price of 30 pens is Rs. 90, how does this affect the loss percentage calculation?

Tip: Always calculate cost price and selling price per unit when dealing with bulk transactions!