A boat covers 216 km downstream in 9 hours and 120 km upstream in 6 hours. If speed of boat in still water in decreased by 50% and speed of stream is increased by 50%, then find the new downstream speed of the boat (in km/hr)?

Correct option is C

​Downstream speed of the boat = $$\frac{216}{9} = 24 \, \text{km/hr}$$ 

Upstream speed of the boat = $$\frac{120}{6} = 20 \, \text{km/hr}$$ 

Speed of boat in still water = $$\frac{24 + 20}{2} = 22 \, \text{km/hr}$$ 

Speed of stream = 24 – 22 = 2 km/hr 

​New speed of boat in still water = $$\frac{22}{2} = 11 \, \text{km/hr}$$ 

And, new speed of stream = $$\frac{2 \times 3}{2} = 3 \, \text{km/hr}$$ 

​New downstream speed = 11+3 =14 km/hr