Watson buys a book for Rs. 240 and sells it to Johny at a profit of 50 percent. Johny wants to sell the same book to Shekar so that he earns a profit of 25 percent, but Shekar insists on buying it at a discount of 10 percent on the marked price. What marked price should Johny quote so that he still achieves his desired profit of 25 percent?

Introduction / Context:
This problem combines successive profits and a discount, a common pattern in profit and loss aptitude questions. First one person makes a profit and sells to another, and then the second person wants a further profit while giving a discount to the final buyer. The question asks for the marked price that allows the second person to keep a desired profit percentage despite allowing a discount.

Given Data / Assumptions:

• Watson buys the book for Rs. 240.
• Watson sells it to Johny at a profit of 50 percent.
• Johny wants a profit of 25 percent on his own cost price.
• Shekar will buy only if he gets a discount of 10 percent on the marked price.
• We must find the marked price that Johny should quote.

Concept / Approach:
We need to move step by step through the chain of transactions. First, we calculate Johny cost price from Watson selling price. Next, we find the selling price that gives Johny a 25 percent profit. Finally, we relate that selling price to the marked price after providing a 10 percent discount. Throughout, we use the basic profit formula: selling price equals cost price multiplied by (1 plus profit percentage divided by 100). For discounts, selling price equals marked price multiplied by (1 minus discount percentage divided by 100).

Step-by-Step Solution:
Watson cost price is 240 rupees.Watson sells with a profit of 50 percent, so his selling price equals 240 * 1.50 which is 360 rupees.Therefore, Johny cost price is 360 rupees.Johny wants a profit of 25 percent on his cost, so required selling price equals 360 * 1.25 which is 450 rupees.Shekar receives a discount of 10 percent on the marked price, so actual selling price equals 90 percent of the marked price.Let marked price be M, then 0.90 * M equals 450.Therefore M equals 450 / 0.90 which is 500 rupees.

Verification / Alternative check:
We can quickly verify the result. If Johny marks the price at 500 rupees and gives a 10 percent discount, the sale happens at 500 * 0.90 equals 450 rupees. His cost price is 360 rupees, so his profit is 450 minus 360 equals 90 rupees. Percentage profit equals 90 / 360 * 100 which is exactly 25 percent, matching the requirement. This confirms that our calculations are consistent at every stage of the transaction chain.

Why Other Options Are Wrong:
Option 450 rupees would become 405 rupees after a 10 percent discount, giving Johny a profit of only 45 rupees, which is 12.5 percent, not 25 percent. Option 360 rupees would give a selling price of 324 rupees after discount, which is even below Johny cost price, leading to a loss. Option 415 rupees would give a selling price of 373.5 rupees, which yields a profit of only 13.75 rupees, again far below 25 percent. Only 500 rupees satisfies the discount and profit conditions together.

Common Pitfalls:
A common mistake is to apply the 25 percent profit directly on Watson original cost instead of on Johny cost, which changes the base. Another mistake is to ignore the discount and simply increase the price by 25 percent, which does not ensure that the final discounted selling price gives the desired profit. Some learners also confuse marked price with selling price. Remember that the discount applies on the marked price and the profit is always computed on cost price.

Final Answer:
Johny should mark the book at Rs. 500 in order to give a 10 percent discount and still earn a 25 percent profit.