A boat takes 2 hours to go 16 km downstream; while covering the same distance upstream, it takes 4 hours. What is the speed of the current?

Let speed of boat in still water $=x$ km/h and speed of current $=y$ km/h.Downstream speed $=x+y=\frac{\text{Distance}}{\text{Time}}=\frac{16}{2}=8$ km/h.Upstream speed $=x-y=\frac{\text{Distance}}{\text{Time}}=\frac{16}{4}=4$ km/h.Subtracting the equations:
$$(x+y)-(x-y)=8-4$$
$$2y=4\Rightarrow y=2$$
The speed of the current is 2 km/h.