If the boats travel about $4km$ in an hour then how long will they take to go a distance of $10km$?

Given:

Distance covered in $1hr = 4km$

To find: the time taken to cover $10km$

First, we have to find the Speed of the boat

We know that

$Speed = \frac{Dis\tan ce}{Time}$

So, the Speed of the boat $=\frac{4}{1}=4km/hr$

Now, time taken to cover $10 km = \frac{dis\tan ce}{speed}=\frac{10}{4}hr$

                                                              $=\frac{5}{2}hr$

                                                         $=\frac{5}{2}\times 60 min$

                                                       $=150 min$

                                                       $= 2hr 30min$

Hence, option B is the correct answer.