A boat running downstream covers a distance of 16 km in 2 hours while for covering the same distance upstream it takes 4 hours. What is the speed of the boat in still water?

Boat in downstream covers a distance of 16 km in 2 hours 

It covers distance of 16km upstream in 4 hours. 

So, rate of downstream $=\left(\frac{16}{2}\right)\mathrm{kmph}=8\mathrm{kmph}$

Also, rate of upstream  $=\left(\frac{16}{4}\right)\mathrm{kmph}=4\mathrm{kmph}$

Thus, speed in the still water can be given as,

$$\begin{gathered} =\frac{1}{2}(8+4) \mathrm{kmph} \\ =6 \mathrm{kmph} \end{gathered}$$

Hence, option (B) is correct.