Number of Dell laptops sold by store T is what percent more than the number of laptops sold by store P?

Answer: Option C

Number of Dell laptops sold by store T
$$\begin{array}{rl}& =2400\times \frac{21}{100}\\ & =504\end{array}$$
Number of Laptops sold by store P
$$\begin{array}{rl}& =2400\times \frac{15}{100}\\ & =360\end{array}$$
Required Percentage
$$\begin{array}{rl}& =\frac{504-360}{360}\times 100\\ & =40\%\end{array}$$