To go a distance of 40 km upstream a rower takes 8 hours while it takes only 5 hours to row the same distance down stream. What was the rower’s speed in still water?

Let the speed of the boat be x

And the speed of the stream be y

Downstream:

Speed = ( x + y) km/h

Distance = 40 km

Time = 5 hours

Distance = Speed x Time

40 = 5( x + y)

Upstream:

Speed = ( x - y) km/h

Distance = 40 km

Time = 8 hours

Distance = Speed x Time

40 = 8(x - y)

Putting the two equations together:

40 = 5( x + y) ----------------------- [ 1 ]

40 = 8 (x - y) ----------------------- [ 2 ]

From [ 1 ]:

40 = 5x + 5y 

8=x+y----------------------- [ 3 ]

From [ 2 ] :

4 0= 8 (x - y)

40 = 8 x - 8 y

5 = x - y    ----------------------- [ 4 ]

[ 3 ] + [ 4 ]:

13 = 2x

x = 6.5 km/h ----------- sub into [ 3 ] to find y

Find y :

8 = 6.5 + y

y = 1.5 km/h

The rowers speed in still water is nothing but the speed of the boat = 6.5 km/h