A boat takes 90 minutes less to travel 36 km downstream than to trave…

The correct answer is option (3) : 2 km/h

$d= 36km, t = 90\, min = 1.5\, hours$

$x= 10\, km/h$

$∵d=\frac{t(x^2-y^2)}{2y}$

$36=\frac{1.5(10^2-y^2)}{2y}$

$ 48y = 100-y^2$

$y^2+48y-100=0$

$y^2+50y-2y-100=0$

$y(y+50)-2(y+50)=0$

$(y-2)(y+50)=0$

$y =-50, 2 $

The value y cannot be negative

So, the speed of the stream is 2 km/h