Correct Answer - Option 4 : 50 minutes
Given:
Speed downstream is 3 times speed upstream.
In still water boat goes 50 m in 5 seconds.
Concept:
Speed downstream is sum of the speed of boat in still water and speed of stream
Speed upstream is difference of the speed of boat in still water and speed of stream.
To convert hours to minutes we have to multiply by 60
Formula:
Speed = distance/time
Time = distance/speed
Speed in m/s × 18/5 = speed in km/h
Calculation:
Speed of boat in still water = 50/5 = 10 m/s
Let the speed of stream be x m/s
Speed during downstream = (10 + x) km/h
Speed during upstream = (10 – x) km/h
3 × (10 – x) = (10 + x)
⇒ (10 – 3x) = (10 + x)
⇒ 3x + x = 30 – 10
⇒ 4x = 20
⇒ x = 5
Speed downstream = 10 + 5 = 15 m/s
Speed downstream = 15 × (18/5) = 54 km/h
Time to cover 45 km = 45/54 = (5/6) hours
Time taken to cover 45 km = (5/6)× 60 = 50 mins
∴ Time to cover the distance downstream is 50 minutes
