A man purchased a watch for Rs. 400 and sold it at a gain of 20% of the selling price. The selling price of the watch is:__________?

A. Rs. 300
B. Rs. 320
C. Rs. 440
D. Rs. 500
Explanation:
Let S.P = Rs. X
Then, 400 + 20% of x = x
(Or) 400 + X/5 = X
(Or) 4X/5 = 400
(or) x = 400 × 5/4 = 500
S.P = Rs. 500