1) A motor boat goes 15 km against the stream running at 4 km/h and back in 2 hours. The speed in km/h of the motorboat in still water is A) 16 B) 15 C) 13.5 D) 18 E) 19 Answer: Option AExplanation:Let speed of motor boat in still water $=x$ km/h Downstream speed $=(x+4)$ km/h Upstream speed $=(x-4)$ km/h $\frac{15}{x-4}+\frac{15}{x+4}$ $=2$ $x$ = 16, -1 Speed cannot be negative, so $x$ = 16 km/h
