Answer : Option C

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{average speed = }\dfrac{2 \times 64 \times 80}{64 + 80} = \dfrac{2 \times 64 \times 80}{144} = \dfrac{2 \times 32 \times 40}{36}\\\\
&= \dfrac{2 \times 32 \times 10}{9} = \dfrac{64 \times 10}{9} = 71.11\text{ kmph} \end{align} $MF#%

$MF#%\text{Time taken to travel first 160 km = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{160}{64}
$MF#%

$MF#%\begin{align}
&\text{Time taken to travel next 160 km = }\dfrac{\text{distance}}{\text{speed}} = \dfrac{160}{80}\\\\\\\\\\\\\\\
&\text{Total distance traveled = }160 + 160 = 2 \times 160\\\\
&\text{Total time taken = }\dfrac{160}{64} + \dfrac{160}{80}\\\\
&\text{Average speed = }\dfrac{\text{Total distance traveled}}{\text{Total time taken}} = \dfrac{2 \times 160}{\dfrac{160}{64} + \dfrac{160}{80}}\\\\\\\\
&= \dfrac{2}{\dfrac{1}{64} + \dfrac{1}{80}} = \dfrac{2 \times 64 \times 80}{80 + 64}= \dfrac{2\times 64 \times 80}{144} = \dfrac{2 \times 8 \times 80}{18}= \dfrac{640}{9}\\\\
&= 71.11\text{ km/hr}
\end{align} $MF#%