<PRE><B>Question 726623</B>
I presume 31 km is up and down each way

boat   	speed	x   	kph														
current     	speed	5	kph														
																	
against	current     	x-	5	kph													
with	current     	x+	5	kph													
																	
Distance=	31	miles															
																	
Time against + time with  =			11	hours													
t=d/r																	
																	
31	/(	x   	+	5	)	+	31	/(x	-	5	)	=	11				
																	
LCD =	(x	-	5	)	(	x   	+	5	)								
31	*(	x   	-	5	)	+	31	(x	+	5	)	=	11	(x^2	-	25	)
31	x   	-	-155	+	31	x   	+	155		=	11	(	x ^2		-	25	)
62	x   	=	11	x ^2		-275											
11	x ^2	-	-62	x   	-	275											
																	
Find the roots of the equation by quadratic formula																	
																	
a=	11	,	b=	-62	,	c=	-275										
																	
b^2-4ac=	3844	+	12100														
b^2-4ac=	15944																
{{{	sqrt(	15944	)=	126.27	}}}												
{{{x=(-b+-sqrt(b^2-4ac))/(2a)}}}																	0
{{{x1=(-b+sqrt(b^2-4ac))/(2a)}}}																	
x1=(	62	+	126.27	)/	22												
x1=	8.56																
{{{x2=(-b-sqrt(b^2-4ac))/(2a)}}}																	
x2=(	62	-126.27	) /	22													
x2=	-2.92																
Ignore negative value																	
boat   	speed	8.56	kph														
																	
m.ananth@hotmail.ca																	
</PRE>