Question

It took a crew 3 h 20 min to row 5 km upstream and back again. if the rate of flow of the stream was 2 km/h, what was the rowing speed of the crew in still water?

Answer 1

Suppose rowing speed is V km/s.

So, upstream speed = V-2

Downstream speed = V+ 2

Total time is taken 3 h 20 min to cover up and down journey of 5 km.

So, total time = upstream time + downstream time

`3. 3 \ hours  = (5 km)/( V- 2) + (5 km)/(V+2) \\\\\ 3.3 (V^2-2^2) = 10V\\\\V^2- 4-3V= 0`

`V^2 -4V+V -4=0 \\\\\ V( V-4) +1 (V-4)=0`

`V= 4 km/h` and
`V= -1 \  km/h`, we take only positive velocity.

Thus, the the rowing speed of the crew in still water is 1 km/s.