Question 807432
Boat speed	=	15	km/h										
current speed	=	x	km/h										
													
speed against	current	15	-	x									
speed with	current	15	+	x									
													
Distance against	current	35	miles										
Distance with	current	140	miles										
													
t=d/r	time against = time with		current	-									
35	/	(	15	-	x	)=	140	/	(	15	+	x	)
35	(	15	+	x 	)=	140	(	15	-x	)			
525	+	35	x	=	2100	-	-140	x					
140	x	35	x		2100	+	-525						
175	x	=	1575										
/	175												
x	=	9	km/h	current	speed