Answer : Option C

Explanation :

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Solution 1 (Quick)
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$MF#%\begin{align}
&\text{If a car covers a certain distance at x kmph and an equal distance at y kmph. Then,}\\
&\text{the average speed of the whole journey = }\dfrac{2xy}{x+y}\text{ kmph.}\\\\\\\\\\\\
&\text{By using the same formula, we can find out the average speed quickly}\\\
&\text{average speed = }\dfrac{2 \times 50\times 30}{50 + 30} = \dfrac{2\times 50 \times 30}{80} = \dfrac{2 \times 50 \times 3}{8}\\\\
&= \dfrac{50 \times 3}{4} = \dfrac{25\times 3}{2} = \dfrac{75}{2} = 37.5
\end{align} $MF#%
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Solution 2 (Fundamentals)
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Though it is a good idea to solve the problems quickly using formulas, you should
know the fundamentals too. Let's see how we can solve this problems using basics.
$MF#%\begin{align}
&\text{Total time taken for traveling one side = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{150}{50}\\\\
&\text{Total time taken for return journey = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{150}{30}\\\\\\\\\\\\\\\
&\text{Total distance travailed = }150 + 150 = 2 \times 150\\\\
&\text{Total time taken = }\dfrac{150}{50} + \dfrac{150}{30}\\\\
&\text{Average speed = }\dfrac{\text{Total distance traveled}}{\text{Total time taken}} = \dfrac{2 \times 150}{\dfrac{150}{50} + \dfrac{150}{30}}\\\\\\\\
&= \dfrac{2}{\dfrac{1}{50} + \dfrac{1}{30}} = \dfrac{2 \times 50 \times 30}{30+ 50}\\\\
&= \dfrac{2\times 50 \times 30}{80} = \dfrac{2 \times 50 \times 3}{8}\\\\
&= \dfrac{50 \times 3}{4} = \dfrac{25\times 3}{2} = \dfrac{75}{2} = 37.5
\end{align} $MF#%