A boat takes 1 hr 30 min less to travel 36 km downstream than to travel the same distance upstream. If the speed of boat in still water is 10 km/h, find the speed of stream.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2 km/hr
$V_b$ = Speed of Boat = 10 km/h
$V_s$ = Speed of stream
Downstream speed = $V_b+V_s=10+V_s$
Upstream speed = $V_b-V_s=10-V_s$
Time upstream = $\frac{36}{10-V_s}$
Time downstream = $\frac{36}{10+V_s}$
$∴\frac{36}{10-V_s}-\frac{36}{10+V_s}=1.5$
$36\left(\frac{2V_s}{(10-V_s)(10+V_s)}\right)=1.5$
$72V_s=150-1.5{V_s}^2$
$V_s=2km/hr$