Boy sells a book for Rs. 720 he gets a loss of 10 %, To gain 10%, what should be the SP?  Choose the correct option.
A) 430 B) 880 C) 550 D) 590 E) 600

Rationale: "cost price = 720 / 90 x 100 = 800 to gain 10 % = 800 x 10 / 100 = 80 sp = cp + gain = 800 + 80 = 880 answer : b"
Answer: B