Answer : Option B

Explanation :

$MF#%\begin{align}
&\text{Time = 1 hr 40 min 48 sec = }1\text{hr }+\dfrac{40}{60}\text{hr} + \dfrac{48}{3600}\text{hr}
= 1 + \dfrac{2}{3} + \dfrac{1}{75} = \dfrac{126}{75}\text{hr}\\
&\text{distance = 42 km}\\\\
&\text{speed} = \dfrac{\text{distance}}{\text{time}}=\dfrac{42}{\left(\dfrac{126}{75}\right)}=\dfrac{42 \times 75}{126}\\
&\Rightarrow \dfrac{5}{7}\text{ of the actual speed = } \dfrac{42 \times 75}{126}\\\\
&\Rightarrow \text{actual speed = } \dfrac{42 \times 75}{126} \times \dfrac{7}{5} = \dfrac{42 \times 15}{18} = \dfrac{7\times 15}{3} = 7\times 5 = 35\text{ km/hr}\\
\end{align} $MF#%