A retailer buys 40 pens at a marked price of 36 pens from a wholesaler. If he…

Concept: When a trader buys N units but pays only the marked price of M units (M less than N), the effective cost price per unit becomes (M / N) times the marked price. This is the classic quantity based trader trick. Profit percent is then Profit % = ((Selling Price minus Cost Price) / Cost Price) times 100, with cost price as the base.

Application:

1. Let the wholesaler's marked price of one pen be Rs 1 (any convenient unit works since only the percentage is required).

2. The retailer pays for 36 pens but receives 40 pens, so his cost price for 40 pens = 36 times 1 = Rs 36, giving a cost price per pen of 36/40 = Rs 0.9.

3. He resells each pen at a 1% discount on the marked price, so his selling price per pen = 1 minus 1% of 1 = Rs 0.99.

4. Profit per pen = Selling Price minus Cost Price = 0.99 minus 0.9 = Rs 0.09.

5. Profit percent = (Profit / Cost Price) times 100 = (0.09 / 0.9) times 100 = 10%.

Cross-check: Scaling up to 40 pens, total cost = 40 times 0.9 = Rs 36 (matches the 36-pens cost given in the question), total sale = 40 times 0.99 = Rs 39.6, and total profit = Rs 3.6. Then (3.6 / 36) times 100 = 10%, the same result -- confirming the profit percent is 10%.