Question 589633
Boat  	speed	x   	mph														
current     	speed	4	mph														
																	
against	current     	x-	4	mph													
with	current     	x+	4	mph													
																	
Distance=15	miles															
																	
Time against + time with  =7hours													
t=d/r																	
																	
15/(x+4)	+15/(x-4)=7				
																	
LCD =(x-4)(x+4)								
15*(x-4)	+15(x+4)	=	7				
15x-60+15x+60=7(x ^2-16)
30x=7x ^2-112											
7x ^2-30	x-112											
																	
Find the roots of the equation by quadratic formula																	
																	
a=	7	,	b=	-30	,	c=	-112										
																	
b^2-4ac=	900	+	3136														
b^2-4ac=	4036	
{{{	sqrt(	4036	)=	63.53	}}}									
{{{x=(-b+-sqrt(b^2-4ac))/(2a)}}}																	
{{{x1=(-b+sqrt(b^2-4ac))/(2a)}}}																	
x1=(	30	+	63.53	)/	14												
x1=	6.68																
x2=(	30	-63.53	) /	14													
x2=	-2.39																
Ignore negative value																	
Boat  	speed	6.68	mph														
																	
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