Question 722529
Plane  	speed	=x	mph						
wind   	speed	=y	mph						
against  	wind   	4	hours						
with	wind   	3	hours						
									
Distance against 		1680	miles	distance with 		1680	miles		
t=d/r	against	wind   		-					
1680	/	(	x	-	y	)=	4		
4	(	x	-	y	)	=	1680		
4	x	-	4	y	=	1680	....................1		
									
1680	/	(	x	+	y	)=	3		
3	(	x	+	y	)	=	1680		
3	x	+	3	y	=	1680	...............2		
Multiply (1) by	3    								
Multiply (2) by	4								
we get				y					
12	x	+	-12	y	=	5040			
12	x	+	12		=	6720			
24	x	=	11760						
/	24								
x	=	490	mph						
									
plug value of x in (1)				y					
4	x		-4	y	=	1680			
1960			-4	-1960	=	1680			
-4	y	=	1680						
-4	y	=	-280	mph					
	y	=	70						
Plane  	speed	490	mph						
wind   	speed	70	mph						
									
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