Question 903384
Let speed with current = x
speed against current be y
t=d/r

time downstream + time upstream = total time

4/x +2/y = 1

let 1/x =a and 1/y be y
4a+2b=1.........................(1)

6/x +5/y =2
6a+5b=2.........................(2)

4	a	+	2	b  	=	1	.............1	
								
6	a	+	5	b  	=	2	.............2	
Eliminate	y							
multiply (1)by		-5						
Multiply (2) by		2						
-20	a		-10	b  	=	-5		
12	a	+	10	b  	=	4		
Add the two equations								
-8	a				=	-1		
/	-8							
a	=	0.125	

x=1/a=1/0.125=	8 
speed of boat = 8 mph				
plug value of			a	in (1)				
4	a	+	2	b  	=	1		
1		+	2	b  	=	1		
			2	b  	=	1		-1
			2	b  	=	1		
				b  	=	0.25	

b=0.25 y=1/b =1/0.25=	4

speed of current = 4 mph