A shopkeeper offers a 5% discount on a watch. If he instead offers a 7% discount, his profit decreases by Rs 15. Find the marked price of the watch.

Introduction:
When a seller changes the discount rate, the change in selling price equals the change in discount taken on the marked price. If this change translates directly to a change in profit (cost remaining the same), the rupee difference reveals the marked price.

Given Data / Assumptions:

• Two scenarios: 5% discount and 7% discount.
• Profit difference between scenarios = Rs 15 (lower profit at 7%).
• Cost price is unchanged across scenarios.

Concept / Approach:
Let M be the marked price. The price drop when moving from 5% to 7% discount equals 2% of M. This drop causes the profit to fall by Rs 15. Hence 0.02M = 15, from which M can be found.

Step-by-Step Solution:
Change in discount rate = 7% - 5% = 2%2% of M = Rs 15M = 15 / 0.02 = 750

Verification / Alternative check:
At 5% discount, SP = 0.95M = 712.50. At 7% discount, SP = 0.93M = 697.50. The difference is Rs 15, which matches the stated profit change when cost is fixed.

Why Other Options Are Wrong:

• Rs. 697.50 and Rs. 712.50: these are selling prices under 7% and 5% discounts, not the marked price.
• Rs. 787.50: not aligned with a 2% difference of Rs 15.

Common Pitfalls:

• Applying the 2% to selling price rather than to the marked price.
• Assuming cost price changes between scenarios.

Final Answer:
Rs. 750.00