Speed of boat downstream is 20 + 5  = 25 km/hr

Speed of boat upstream is 20  - 5 = 15 km/hr

Time downstream = Distance Downstream/25

Time upstream = return distance (same as Distance Downstream)/15

Time downstream + Time upstream = 3 hours.

Therefore D/25 + D/15 = 3

Multiply both sides by 225, giving 9D + 15D = 675

24D = 675

D = 28.12

Time downstream = 28.12/25 = 1.12 hrs.

So he should turn around after going downstream for 1.12 hours.

See first answer for a distance/time graph.

The boat travels at 20 km/h in still water. When it leaves its base it travels downwards. As the river flows at 5 km/h, the speed of the boat downstream is 25 km/h. The speed of the boat upstream is 15 km/h.

Let the time when the boat has to turn around be T hours after starting. The distance traveled downstream is 25*T. After turning around the distance travels upstream is 15*(3 - T). These have to be equal to allow the boat to return back.

25*T = 45 - 15*T

=> 40*T = 45

=> T = 45/40 hours

The distance-time graph as it travels downstream and upstream is:

The red line shows the journey downstream and the green line shows the journey upstream.

The boat should turn back after 1.125 hours.