A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is: Choose the correct choice:
A) 10  B) 6  C) 5  D) 4  E) 3
Explanation :   Speed of the motor boat = 15 km/hr   Let speed of the stream = v   Speed downstream = (15+v) km/hr   Speed upstream = (15-v) km/hr   Time taken downstream = 30/(15+v)   Time taken upstream = 30/(15−v)   total time = (30/(15+v)) + (30/(15−v))   It is given that total time is 4 hours 30 minutes = 4(1/2) hour = 9/2 hour   i.e.,(30/(15 + v)) + (30/(15 − v)) = 9/2   ⇒ (1/(15+v)) + (1/(15−v)) = 9/(2×30) = 3/20   ⇒ (15 − v + 15 + v)/(15+v)(15−v) = 3/20   ⇒ (30/152)− (v2) = 3/20   ⇒ (30/225−v2) = 3/20   ⇒ 10/(225−v2) = 1/20   ⇒225 − v2= 200   ⇒ v2 = 225 − 200 = 25   ⇒ v = 5 km/hr. Answer : Option 5
<result>C</result>
A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is: