Answer : Option D

Explanation :

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Solution 1 (Recommended)
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$MF#%\boxed{\text{Speed and time are inversely proportional }\\
\Rightarrow \text{Speed ∝ }\dfrac{1}{\text{Time}}\text{ (when distance is constant)}}$MF#%

$MF#%\begin{align}
&\text{Here distance is constant and Speed and time are inversely proportional}\\
&\text{Speed ∝ }\dfrac{1}{\text{Time}}\\\\
&\Rightarrow \dfrac{\text{Speed1}}{\text{Speed2}} = \dfrac{\text{Time2}}{\text{Time1}}\\\\
&\Rightarrow \dfrac{240}{\text{Speed2}} = \dfrac{\left(1\dfrac{2}{3}\right)}{5}\\\\
&\Rightarrow \dfrac{240}{\text{Speed2}} = \dfrac{\left(\dfrac{5}{3}\right)}{5}\\\\
&\Rightarrow \dfrac{240}{\text{Speed2}} = \dfrac{1}{3}\\\\
&\Rightarrow \text{Speed2} = 240 \times 3 = 720\text{ km/hr}
\end{align} $MF#%

$MF#%\begin{align}
&\text{New time = }1\dfrac{2}{3}\text{ hr} = \dfrac{5}{3}\text{ hr}\\\\
&\text{Hence, new speed = }\dfrac{\text{Distance}}{\text{Time}} = \dfrac{240 \times 5}{\dfrac{5}{3}} = 240 \times 3 = 720\text{ km/hr}\\
\end{align} $MF#%