A boat can travel 5 km upstream in 15 minutes. If the ratio of the speed of the boat in still water to the speed of the stream is 6 : 1, then how much time will the boat take to cover 19.6 km downstream ?

42 minutes

Upstream Speed = Speed of Boat – Speed of current

Downstream Speed = Speed of Boat + Speed of current

Given,

A boat can travel 5 km upstream in 15 minutes.

The ratio of the speed of the boat in still water to the speed of the stream is 6 : 1

Let the speed of the boat = 6x and the speed of the stream = x

Then, according to the question,

Upstream speed = 6x - x = 5x

= 5x = (5 × 60)÷15

= 5x = 20

= x = 4 km/hr

Downstream speed = 24 + 4 = 28 km/h

Downstream distance = 19.6 km

Time = \(\frac{19.6}{28}\) = 0.7 h. = 42 min.