Mohit buys an old bicycle for Rs. 2700 and spends Rs. 500 on its repairs. If he sells the bicycle for Rs. 3520, what is his profit percentage on the total cost?

Introduction / Context:
This question checks whether you correctly include all cost components when computing profit percentage. Mohit buys a second-hand bicycle and spends additional money on repairs. The true cost price is the sum of the purchase price and the repair expenses. Ignoring the repair cost is a very common mistake in such problems. Once you form the correct total cost, the question is a straightforward profit percentage calculation.

Given Data / Assumptions:
- Purchase price of the old bicycle = Rs. 2700. - Repair cost incurred by Mohit = Rs. 500. - Selling price of the bicycle after repairs = Rs. 3520. - There are no other hidden costs or taxes. - We are asked to find the profit percentage on the total cost.

Concept / Approach:
The central concept is that cost price should include all expenditure made to get the asset into saleable condition. Thus, cost price CP = purchase price + repair cost. Profit is defined as Profit = Selling Price - Cost Price. After obtaining the absolute profit, we calculate profit percentage using Profit Percentage = (Profit / Cost Price) * 100. These formulas are standard in profit and loss problems and must be applied systematically.

Step-by-Step Solution:
Step 1: Compute total cost price CP. Step 2: CP = 2700 + 500 = Rs. 3200. Step 3: Given selling price SP = Rs. 3520. Step 4: Profit = SP - CP = 3520 - 3200 = Rs. 320. Step 5: Profit percentage = (Profit / CP) * 100. Step 6: Profit percentage = (320 / 3200) * 100. Step 7: Simplify: 320 / 3200 = 0.10, so profit percentage = 10%.

Verification / Alternative check:
We can also check by scaling. If the cost price is 3200, then 10% of 3200 is 320. Adding this to the cost gives a selling price of 3520, which exactly matches the given selling price. Therefore the profit percentage must be 10%. If we had mistakenly used only 2700 as cost, the profit would seem to be 820, leading to a much higher and incorrect percentage, which shows why including repair cost is essential.

Why Other Options Are Wrong:
A profit of 12.5%, 15%, or 20% on a cost of 3200 would lead to profits of 400, 480, or 640 respectively, resulting in selling prices of 3600, 3680, or 3840. None of these match the actual selling price of 3520. These incorrect options often trap students who forget the repair cost or miscalculate the percentage. Only 10% keeps the cost and selling price relationship consistent.

Common Pitfalls:
The biggest pitfall is forgetting to add the repair cost to the initial purchase price. Students sometimes treat repair cost as something separate and compute profit only on the original purchase price, which is not correct from a business perspective. Another mistake is computing percentage on the selling price instead of the cost price. Always remember: unless clearly stated otherwise, profit and loss percentages are based on cost price.

Final Answer:
Mohit makes a 10% profit on the bicycle when he sells it for Rs. 3520 after spending on repairs.