A boat can row 35 km, upstream in 7 hours and the same distance downstream in 5 hours. What is the speed (in km/hr) of the boat in still water?

Answer: Option D

Speed of boat in upstream = $$\frac{35}{7}$$ = 5 km/hr
Speed of boat in downstream = $$\frac{35}{5}$$ = 7 km/hr
Let the speed of boat x and speed of stream be y
Now,
x - y = 5
x + y = 7
$$\overline{2\text{x}=12}$$
∴ x = 6 km/hr