Answer : Option D

Explanation :

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Solution 1 (Quick)
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$MF#%
\boxed{\text{If a car covers a certain distance at x kmph and an equal distance at y kmph,}\\
\text{the average speed of the whole journey = }\dfrac{2xy}{x+y}\text{ kmph.}}\\
\begin{align}
&\text{By using the same formula, we can find out the average speed quickly}\\\
&\text{Speed with which he travels from A to B = x = 40 km/hr}\\
&\text{Speed with which he travels from B to A = x = }40 \times \dfrac{(100 + 50)}{100} = 60\text{ km/hr}\\
&\text{average speed = }\dfrac{2 \times 40 \times 60}{40 + 60} = 48\text{ km/hr}\\\\
\end{align} $MF#%

$MF#%\begin{align}&\text{Time to travel from A to B = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{x}{40}\text{ hr}\\
&\text{Speed with which he travels from B to A = }40 \times \dfrac{(100 + 50)}{100} = 60\text{ km/hr}\\
&\text{Time to travel from B to A = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{x}{60}\\\\\\\\\\\\\\\
&\text{Total distance traveled = }x + x= 2x\\\\
&\text{Total time taken = }\dfrac{x}{40} + \dfrac{x}{60}\\\\
&\text{Average speed = }\dfrac{\text{Total distance traveled}}{\text{Total time taken}} = \dfrac{2x}{\dfrac{x}{44} + \dfrac{x}{60}}\\\\\\\\
&= \dfrac{2}{\dfrac{1}{60} + \dfrac{1}{40}} = \dfrac{2 \times 2400}{40 + 60}= 2\times 24 = 48\text{ km/hr}\\\\
\end{align} $MF#%