If motor boats travel about $15 km$ in  $5 hours$ then how long will they take to go a distance of $30 km$?

Given:
Distance covered by motor boat$=15km$
Time taken to cover $15km=5hours$

To find: The time taken when it covers $10km$
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 $=\frac{15}{5}=3km/hr$

Now, time taken to cover $30 km = \frac{dis\tan ce}{speed}=\frac{30}{3}hr$

                                                              $=10hr$

Hence, option C is the correct answer.