Boat speed in still water

Question

Jane took 20 minutes to drive her boat upstream to water ski at her favorite spot. Coming back later in the day and the same boat speed, took her 15 minutes. If the current in that part of the river is 2 km per hour, what was her boat speed in still water? 

Answer 1

Let's assign "x" to be the boat speed in km/hour.

 

Then,  "x-2" is the upstream speed, "x+2" is the downstream speed.

 

As the distance covered upstream and downstream is the same, we can write:

 

1/3 (hour) * (x-2) (km/hour) = 1/4 (hour) * (x+2) (km/hour)

 

1/3x - 2/3 = 1/4x +1/2

 

1/3x - 1/4x = 1/2 + 2/3

 

1/12x = 7/6    x = 14 km/hour