Irfaan loves black coffee. A roadside stall sells a cup of black coffee for Rs 120 but offers a 25% discount to Irfaan. If Irfaan is actually willing to pay Rs 200 for this cup, what is his consumer surplus from this purchase?

Introduction / Context:
Consumer surplus is a key concept in microeconomics that measures the extra benefit consumers receive when they pay less for a good than the maximum amount they are willing to pay. It is widely used to analyse welfare effects of price changes, taxes, subsidies and discounts. This question uses a simple everyday example of buying black coffee at a discount to test whether the learner can compute consumer surplus correctly using basic arithmetic.

Given Data / Assumptions:
- Marked price of one cup of black coffee is Rs 120.
- Irfaan receives a 25% discount on this price.
- Irfaan is willing to pay up to Rs 200 for the cup.
- We assume he buys the coffee once at the discounted price.
- We are asked to find his consumer surplus from this single purchase.

Concept / Approach:
Consumer surplus is defined as the difference between the maximum amount a consumer is willing to pay for a good and the amount actually paid. In this question, the maximum willingness to pay is given directly as Rs 200. The actual price paid is the discounted price, which must be calculated from the original price and the discount rate. Once both values are known, the consumer surplus is simply the difference between them.

Step-by-Step Solution:
Step 1: Compute the discount in rupees. A 25% discount on Rs 120 equals 0.25 * 120 = Rs 30. Step 2: Subtract the discount from the original price to get the actual price paid. Actual price paid = 120 - 30 = Rs 90. Step 3: Identify the maximum amount Irfaan is willing to pay. It is given as Rs 200. Step 4: Use the consumer surplus formula: consumer surplus = willingness to pay minus actual price paid. Step 5: Substitute values: consumer surplus = 200 - 90 = Rs 110. Step 6: Therefore, Irfaan's consumer surplus from this purchase is 110 rupees.

Verification / Alternative check:
As a quick check, note that paying Rs 90 for something valued at Rs 200 is clearly a good deal. The difference of Rs 110 represents the extra satisfaction or benefit over and above the amount paid. None of the other options produce this exact difference, which confirms that Rs 110 is the right answer. The units also make sense since both willingness to pay and price are in rupees.

Why Other Options Are Wrong:
90 is wrong because it reuses the price paid, not the surplus. It confuses the benefit with the amount paid.
80 is wrong because it might come from mistakenly calculating discount plus some extra amount instead of directly subtracting price from willingness to pay.
30 is wrong because that is just the size of the discount, not the consumer surplus. Discount and surplus are different concepts.

Common Pitfalls:
Students sometimes confuse the discount received with consumer surplus. The discount is the difference between the original marked price and the discounted price, while consumer surplus compares the discounted price to the maximum willingness to pay. Another common mistake is to miscalculate the percentage discount, especially by subtracting 25 rupees instead of computing 25% of 120. Carefully separating these steps and applying the correct formula will help avoid such errors in similar questions.

Final Answer:
Irfaan's consumer surplus from this cup of black coffee is Rs 110.