If I purchased 11 books for Rs. 10 and sold all at the rate of 10 books for Rs. 11, the profit percent

A. 10%
B. 11%
C. 21%
D. 100%
Explanation:
Suppose I Purchased 110 books
C.P = Rs. (10/11 × 110) = Rs. 100
S.P = Rs. (11/10 × 110) = Rs. 121
Gain % = 21%