While selling a watch, a shopkeeper gives a discount of 15%. If he gives a discount of 20% he earns Rs.51 less in profit. What is the original price of the watch?

Let the marked price be P. Raising the discount from 15% to 20% lowers the selling price, and so the profit, by 5% of P. So 0.05P = Rs 51 and P = Rs 1,020. Rs 1,000 is wrong because 5% of it is only Rs 50.