UPSC 2016 CDS II Math Paper

Let the speed of the boat in still water be x km/hour and the speed of the current be y km/hour.
Distance covered by the boat downstream in 2 hours = 20 km
∴(x+y)×2=20
⇒x+y=10....(1)
Distance covered by the boat upstream in 2 hours = 4 km
∴(x−y)×2=4
⇒x−y=2.....(2)
Subtracting (2) from (1), we have
2y=8
⇒y=4
Thus, the speed of the current is 4 km/hour.