The speed of a boat down the stream is 125% of the speed in still water. If the boat takes 30 minutes to cover 20 km in still water, then how much time (in hours) will it take to cover 15 km upstream?

Question: The speed of a boat down the stream is 125% of the speed in still water. If the boat takes 30 minutes to cover 20 km in still water, then how much time (in hours) will it take to cover 15 km upstream?

Solution:
Given that,
Speed downstream = 125% of speed in still water
Distance in still water = 20 km, time = 30 min = 0.5 hr
Distance to travel upstream = 15 km

Speed of boat in still water = Distance/Time = 20/0.5 = 40 km/h
And,
Downstream speed = 125% of still water speed.
∴ Downstream speed = 1.25 × 40 = 50 km/h

We know,
Downstream speed = Boat speed in still water + Current speed
⇒ 50 = 40 + Current speed
⇒ Current speed = 50 - 40 = 10 km/h

And upstream speed = Boat speed in still water - Current speed = 40 - 10 = 30 km/h

∴ Time to cover 15 km upstream = Distance/Speed ​= 15/30 = 1/2 = 0.5 hours

So the time required for the boat to cover 15 km upstream is 0.5 hours.