The marked price of a watch was Rs. 720. A man bought the same for Rs. 550.80, after getting two successive discounts, the first 10%. What was the second discount rate ?

Answer: Option C

Market price of a watch = Rs. 720
After 1^st discount
$$\begin{array}{rl}& \text{ = 90}\%\text{ of 720 }\\ & \text{ = }\frac{90}{100}\times 720\\ & =\text{Rs}\text{. 648}\end{array}$$
Cost price of watch = Rs. 550.80
Difference
= 648 - 550.80
= Rs. 97.20
$$\begin{array}{rl}& {\text{2}}^{\text{nd}}\text{ discount}\\ & =\frac{97.20}{648}\times 100\\ & =\frac{9720}{648}\\ & =15\%\end{array}$$