A boat has speed a of 15 m s in still water but while going upstream its speed is half the speed as ...........

Quantitative Aptitude(IBPS RRB PO) Boats and Streams

A boat has speed a of 15 m/s in still water, but while going upstream its speed is half the speed as compared to the speed of the boat going downstream. The boat is going downstream and a ship while going downstream crosses the boat in 15 seconds. The length of the ship is double the length of the boat. Find the time the ship will take to cover a distance of 144 kilometers while going upstream if the length of the ship is 100 meters.

This questions was previously asked in

RRB Officer Scale-I Mains (1st Oct 2022)

View All RRBA Previous Year Papers

1 hour

2 hours

1.33 hours

1.6 hours

3 hours

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Explanation:

Correct option 2: 2 hours

Let speed of stream be x
Speed of boat downstream = (15+ x) m/s
Speed of boat upstream = (15 - x) m/s
15+ x = 2(15-x)
= 3x = 15
⇒x=5
Speed of stream = 5 m/s
⇒ Speed of boat during downstream = 15+ 5 = 20 m/s
Length of boat = 100/2 = 50 metres
⇒ Total distance covered by ship to cross the boat = 100 + 50 = 150 metres
Let the speed of the ship be s
⇒ Relative speed = (s-20) m/s
⇒ Speed = 150/15 = 10 m/s
⇒s-20=10
?s = 30 m/s
Speed of ship in still water 30-5 = 25 m/s
Speed of ship upstream 25-5= 20 m/s
20 m/s = 20 x 18/5 = 72 km/hr
Time = 144/72
Time taken by the ship is 2 hours