A man can row a boat in still water at a speed of 4.5 km/hr. In a river flowing at the rate of 1.5 km /hr, he starts his journey from a point downstream and comes back to the same point. Then the average speed of the total journey is :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 4 km/hr
$\text{Average speed}=\frac{\text{2×Speed in still water×Speed against current}}{\text{Speed in still water + Speed against current}}$
$V_{down}=4.5+1.5=6km/hr$
$V_{up}=4.5-1.5=3km/hr$
$\text{Average speed}=\frac{2×V_{down}×V_{up}}{V_{down}+V_{up}}$
$=\frac{2×6×3}{6+3}=\frac{2×6×3}{9}$
$=4 km/hr$