Question

A boat travels upstream 20 km in 6 hrs and 18 kms downstream in 4 hrs find the speed of boat in still water

Answer 1

Upstream speed = 20/6 = 10/3 km/hr
Downstream speed = 18/4 = 9/2 km/hr

Let speed of boat in still water be 'x' &&  speed of stream be 'y'
Upstream speed = x-y
Downstream speed = x+y
x - y = 10/3
x + y = 9/2
adding:
2x = 10/3 + 9/2 = 20+27/6 = 47/6
2x = 47/6
x = 47/12 km/hr = 3.9 km/hr = Speed of boat in still water
Hope it helps