A boat goes 30 km upstream in 3 hours and downstream in 1 hour. How much time (in hours) will this boat take to cover 60 km in still water?

Let the speed of the boat in still water = m

Speed of the stream = s

Upstream speed = m - s

$$\frac{30}{3}$$ = m - s

m - s = 10............(1)

Downstream speed = m + s

$$\frac{30}{1}$$ = m + s

m + s = 30...........(2)

Adding (1) and (2),

2m = 40

m = 20

Speed of the boat in still water = 20 km/h

Time required for the boat to cover 60 km in still water = $$\frac{60}{20}$$ = 3 hours

Hence, the correct answer is Option B