Let speed of boat in still water = $$x$$ km/hr and speed of current = $$y$$ km/hr The boat goes 4 km upstream and 4 km downstream in 1 hour
Using, time = distance/speed => $$frac{4}{x+y}+frac{4}{x-y}=1$$ Similarly, $$frac{5}{x+y}+frac{3}{x-y}=frac{55}{60}$$ Let $$frac{1}{x+y}=w$$ and $$frac{1}{x-y}=z$$ => $$4w+4z=1$$ and $$5w+3z=frac{55}{60}$$ Solving above equations, we get : $$w=frac{1}{12}$$ and $$z=frac{1}{6}$$ => $$x+y=12$$ ---------(i) and $$x-y=6$$ -----------(ii) Adding equations (i) and (ii), => $$2x=12+6=18$$ => $$x=frac{18}{2}=9$$ km/hr => Ans - (C)