The current ofa stream runs at the rate of 2 km/hour. A motor boat goes 10 km upstream & back again to the starting point in 55 min. Find the speed of the motor boat in still water?
Choose the correct choice from the following answers:
A) 20 km/hr  B) 22 km/hr  C) 28 km/hr  D) 30 km/hr  E) 34 km/hr
Let the speed of the boat in still water =x km/hr   Speed of the current = 2 km/hr   Then, speed downstream =(x+2) km/hr   speed upstream =(x−2) km/hr   Total time taken to travel 10 km upstream and back = 55 minutes =5560 hour = 1112 hour   ⇒10x−2+10x+2=1112 120(x+2)+120(x−2)=11(x2−4) 240x=11x2−44 11x2−240x−44=0 11x2−242x+2x−44=0 11x(x−22)+2(x−22)=0 (x−22)(11x+2)=0 x=22 or −211   Since x cannot be negative, x = 22   i.e., speed of the boat in still water = 22 km/hr   22 km/hr
<result>B</result>
The current ofa stream runs at the rate of 2 km/hour. A motor boat goes 10 km upstream & back again to the starting point in 55 min. Find the speed of the motor boat in still water?