Question 568773
speed of current be x mph

let the distance traveled be y

upstream speed = 16-x
downstream speed = 16+x mph

distance / speed = time

y/(16-x)= 1/3

3y=(16-x)

3y=16-x
3y+x=16-------------------(1)

y/(16+x)= 1/4
4y=(16+x)
4y=16+x
4y-x=16...................(2)

x+3y=16	.............1				
-x+4y=	16	.............2				
Eliminate 
multiply (1)by -4									
Multiply (2) by		3									
-4x-12y	=-64					
-3x+12y	=48					
Add the two equations											
-7x=-16					
/	-7										
x=2.29
plug value of 	x	in (1)							
x+3y=16					
2.29+3y	=16					
3y	=	16-2.29			
3y	=	13.71	
y	=	4.57 miles

speed of current = 2.29 mph