Boats and Streams
 A swimmer's speed downstream is 11 km/h and speed of the stream is 1.5 km/h. Find the upstream speed of swimmer. ?

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Rate of stream = ( Speed downstream  Speed upstream )/2
Correct Option: A
Rate of stream = ( Speed downstream  Speed upstream )/2
⇒ 1.5 = (11  Speed upstream)/2
⇒ 11  Speed upstream = 3
∴ Speed upstream = 11  3 = 8 km/h
 A boat goes 48 km downstream in 20 h. It takes 4 h more to cover the same distance against the stream. What is the speed of the boat in still water ?

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Speed of boat in still water = (Speed downstream + Speed upstream)/2
Correct Option: A
Speed downstream = 48/20 = 2.4 km/h
Speed upstream = 48/24 = 2 km/h
∴ Speed of boat in still water = (Speed downstream + Speed upstream)/2
= (2.4 + 2)/2 = 4.4/2 = 2.2 km/h
 Keshav can row 60 km downstream and 36 km upstream, taking 10 h each time. What is the velocity of the current?

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Velocity of the current = (speed downstream  Speed upstream)/2
Correct Option: D
Speed upstream = Distance/Time = 60/10 = 6 km/h
Speed upstream = Distance/Time = 36/10 = 3.6 km/h
∴ Velocity of the current = (speed downstream  Speed upstream)/2
= (6  3.2)/2 = 1.2 km/h
 What time will be taken by a boat to cover a distance of 128 km along the stream, if speed of a boat in still water is 24 km/h and speed of stream is 8 km/h?

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Downstream speed of boat = Speed of boat in still water + Speed of stream
Required time = Distance/Speed downstreamCorrect Option: A
Downstream speed of boat = Speed of boat in still water + Speed of stream
= 24 + 8 = 32 km/h
∴ Required time = Distance/Speed downstream
= 128/32 = 4 km/h
 A boat goes 12 km upstream in 48 minutes. The speed of stream is 2 km/hr. The speed of boat in still water is ?

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∵ (12 x 60)/48 = (L  2)
Correct Option: C
∵ (12 x 60)/48 = (L  2)
∴ L = 15 + 2 = 17 km/hr