Yolanda's boat has a top speed of 9 miles per hour in still water. While traveling on a river at top speed, she went 10 miles upstream in the same amount of time she went 20 miles downstream. Find the rate of the river current.

Let them rate of the stream be x:

speed of boat and river upstream is:

(9-x) mi/hr

speed of boat and river down stream is:

(9+x) mi/hr

time taken to move upstream is:

time=distance/speed

= (9-x) / 10

time taken to move downstream is

time = (9+x) / 20

since they took the same time, we shall have:

(9-x) / 10 = (9+x) / 20

solving for x we get:

20 (9-x) = 10 (9+x)

180-20x=90+10x

180-90=10x+20x

90=30x

hence

x=3 mi/hr

thus the speed of river current is 3 mi/hr