Let speed of boat = x km/hr ⇒ Downstream speed = (x+1.5) km and upstream speed = (x−1.5) km ∵ Distance travelled is same and speed is inversely proportional to time, ⇒
x+1.5
x−1.5
=
3.5
3
⇒ 3x+4.5=3.5x−5.25 ⇒ 3.5x−3x=4.5+5.25 ⇒
x
2
=9.75 ⇒ x=9.75×2=19.5 ∴ Speed of the boat in still water = 19.5 km/hr