Explanation:

Watson bought the book for Rs. 240 and sold to Johny at a profit of 50%.

S.P = C.P(1 + P%/100)

=> S.P for Watson = C.P for Johny

= 240(1 + 50/100) = 240 x 1.5 = Rs. 360

Let Johny quoted the marked price of the book as Rs. M

We know, SP = M.P(1 - Discount(%)/100)

Here discount = 10% to Shekar,

S.P for Johny = M(1 - 10/100) = 0.9M

But Johny want to earn 25% profit,

=> S.P = C.P(1 + P%/100)

=> 0.9M = 360(1 + 25/100)

=> M = (360x1.25)/0.9

=> M = Rs. 500

Therefore, Johny should quote Rs. 500 as the marked price of the book to get 25% profit and allowing 10% discount to Shekar.

Hence, option 3 is the correct answer.