Question 816215
boat   	speed	x	mph				
current     	speed	3	mph				
..							
against	current     	x	-	3	mph		
with	current     	x   	+	3	mph		
							
Distance against	current     	12					
Distance with	current     	18					
Time = same	equate the time against with time with current						
t=d/r							
12	/(x-	3	)=	18	/(x+	3	)
12	*(x+	3	)=	18	*(x-	3	)
12	x  	+	36	=	18	x	-54
12	x  	-18	x   	=	-54	+	-36
-6	x  	=	-90				
/	-6						
	x=	15	mph				
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