Question

A boat took 4 hours to make a downstream

trip with a current of 6 km/h. The return trip
against the same current took 6 hours. Find
the speed of the boat in still water.

Answer 1

30 km/h

Explanation:

Let X km/h be the speed of the boat in still water

Let the distance covered each way be D km

Relative speed of boat downstream = X + 6

Relative speed of boat upstream = X - 6

Distance covered by boat when going downstream

D = 4(X + 6) = 4X + 24

Distance covered by boat upstream is the same as distance covered downstream
D = 6(X-6) = 6X -36

Setting the two X equations equal to each other gives us

6X - 36 = 4X + 24

Collecting like terms

6X - 4X = 24-(-36)

2X = 60

X = 30 km/hr which is the speed of boat in still water

Check

Downstream speed is 30+6 = 36 km/h and distance traveled downstream in 4 hours = 36 x 4 = 144km

Upstream speed is 30-6 = 24 km/h and it travels for 6 hours. Distance covered = 24 x 6 = 144 miles

So distance covered is same and hence check succeeds