Question 452927
boat speed = x
current speed = y
..
distance = 20 miles
time = d/r

..
downstream speed = 20/(x+y)
upstream speed = 20/(x-y)
..
20/(x+y)=4
4x+4y=20
/4
x+y=5...............1
20/(x-y)=5
5x-5y=20
/5
x-y=4...............2

1	x	+	1	y	=	5	.............1	
1	x	+	-1	y	=	4	.............2	
Add the two equations								
2	x				=	9		
/	2							
x	=	4.50						
plug value of 			x	in (1)				
1	x	+	1	y	=	5		
4.5		+	1	y	=	5		
			1	y	=	5		-4.5
			1	y	=	0.5		
				y	=	0.50