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.

2 km/hr

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 = $\frac{36}{10+V_s}$

$∴\frac{36}{10-V_s}-\frac{36}{10+V_s}=1.5$

$36\left(\frac{2V_s}{(10-V_s)(10+V_s)}\right)=1.5$

$72V_s=150-1.5{V_s}^2$

$V_s=2km/hr$