Question

A man can row at 9km/h in still water. He rows 1.5km upstream and back again. The stream flows at 3km/h. What is his average speed for the whole trip?

Answer 1

Answer: average speed is 8 km/h

Step-by-step explanation: in upstream his speed is 9 km/h -3km/h.

So his speed is 6km/h and he needs time t= s/v= 1.5 km: 6 km/h = 0.25 h

Downstream his speed is 9 km/ + 3km/h = 12km/h and time is

1.5 km:12km/h= 1/8 hours= 0.125 h

Average speed v = s/t = 3km: (0.25 + 0.125 ) h = 8 km/h

Answer 2

Answer:

Step-by-step explanation:

Rowing against the current, he travels 9-3 = 6 km/h.

1.5 km × (1 h)/(6 km) = ¼ h

It takes him ¼ h to row 1.5 km upstream.

Rowing with the current, he travels 9+3 = 12 km/h.

1.5 km × (1 h)/(12 km) = ⅛ h

It takes him ⅛ h to row 1.5 km downstream.

Average speed = (3 km)/(¼+⅛ h) = 8 km/h