A man can row at a speed of 10 text{ km/h} in | vedclass

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(B) Let the distance to the point be $d 	ext{ km}$.Speed in still water $(u)$ = $10 	ext{ km/h}$.Speed of the river $(v)$ = $4 	ext{ km/h}$.Upstrea

Vedclass · Accepted Answer

$8 \frac{2}{5}$

Vedclass · Answer

(B) Let the distance to the point be $d 	ext{ km}$.Speed in still water $(u)$ = $10 	ext{ km/h}$.Speed of the river $(v)$ = $4 	ext{ km/h}$.Upstream speed = $u - v = 10 - 4 = 6 	ext{ km/h}$.Downstream speed = $u + v = 10 + 4 = 14 	ext{ km/h}$.Time taken for upstream journey $(t_1)$ = $\frac{d}{6} 	ext{ hours}$.Time taken for downstream journey $(t_2)$ = $\frac{d}{14} 	ext{ hours}$.Total distance = $2d$.Total time = $t_1 + t_2 = \frac{d}{6} + \frac{d}{14} = \frac{7d + 3d}{42} = \frac{10d}{42} = \frac{5d}{21} 	ext{ hours}$.Average speed = $\frac{	ext{Total distance}}{	ext{Total time}} = \frac{2d}{5d/21} = 2d 	imes \frac{21}{5d} = \frac{42}{5} = 8 \frac{2}{5} 	ext{ km/h}$.

$A$ man can row at a speed of $10 \text{ km/h}$ in still water to a certain upstream point and back to the starting point in a river which flows at $4 \text{ km/h}$. Find his average speed for the total journey (in $\text{km/h}$).

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