Question 914016
Boat speed	=	7	km/h										
current speed	=	x	km/h										
													
speed against	current	7	-	x									
speed with	current	7	+	x									
													
Distance against	current	20	miles										
Distance with	current	50	miles										
													
t=d/r	time against = time with		current	-									
20	/	(	7	-	x	)=	50	/	(	7	+	x	)
20	(	7	+	x 	)=	50	(	7	-x	)			
140	+	20	x	=	350	-	-50	x					
50	x	20	x		350	+	-140						
70	x	=	210										
/	70												
x	=	3	km/h	current	speed