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

Does the problem say y=x+4?