Answer:
12 km/hour
Step-by-step explanation:
Given information: The current in that part of the river is 4 km per hr.
Speed of current = 4 km per hour
Let x be the speed of boat in still water.
Speed of boat in upstream = x-4 km per hour
Speed of boat in downstream = x+4 km per hour
Assume that the distance between Jane's initial position and Jane's favorite spot is D.
Jane took 20 min to drive her boat upstream to water-ski at her favorite spot.

For upstream,

.... (1)
For downstream,
..... (2)
Equate (1) and (2) we get

Isolate variable terms.


Divide both sides by 10.

The value of x is 12. Therefore, the speed of boat in still water is 12 km/hour.