Question

Nihar requires twice as long to row up as to row down the river, If the speed of the stream is $3.5km/hr$. then find Nihar's speed in still water.

• A. $10.5km/hr$
• B. $21km/hr$
• C. $7km/hr$
• D. $14km/hr$

Answer 1

The correct option is A $10.5km/hr$

Let the time taken by Nihar to row down the river = 7 hrs

Time taken to row up the river = 2t hours

Let the speed in still water = x km/h

Downstream $\to$

Distance = D Time = 7 hours

speed = (x + 3.5) km / h

upstream $\to$

Distance = D Time = 2t hours

Speed = (x - 3.5) km/h

$x+3.5=\frac{D}{t}$

$D=(x+3.5)7\cdots \left(1\right)$

$x-3.5=\frac{D}{2t}$

$D=(x-3.5)2t\cdots \left(2\right)$

from equation (1) & equation (2)

$xt+3.5t=2xt-7t$

$xt=10.5t$

$x=10.5$

Speed in still water = 10.5 km/h.