Question

A motorboat takes 3 hours to travel 108 kilometers going upstream. The return trip takes 2 hours going

downstream. What is the rate of the boat in still water and what is the rate of the current?

Answer 1

• 45 km/h and 9 km/h

Explanation:

Let the rate of boat in still water be b and the speed of the current be c

• Speed downstream = b + c
• Speed upstream = b - c

We got equations for time:

• 108/(b - c) = 3 ⇒ b - c = 36
• 108/(b + c) = 2 ⇒ b + c = 54

Add up the equations:

• b - c + b + c = 36 + 54
• 2b = 90
• b = 45 km/h

Then finding the values of c

• c = 45 - 36 = 9 km/h