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A boat covers 20 km in 4 hours downstream and returns upstream in 8 hours, then find the speed of the stream.


1. 2 km/hr
2. 1.5 km/hr
3. 1.25 km/hr
4. 2.25 km/hr

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Correct Answer - Option 3 : 1.25 km/hr

Given:

Distance (D) = 20 km

TD (Time taken in downstream) = 4 hours

TU (Time taken in upstream) = 8 hours

Formula used:

Downstream speed (SD) = Speed of boat + Speed of stream

Upstream speed (SU) = Speed of boat – Speed of stream

TD = DD/SD

Where TD, DD, and SD are time, distance and speed respectively during downstream 

TU = DU/SU 

Where TU, DU, and SU are time, distance and speed respectively during upstream

Calculation:

Let the speed of boat be x km/hr and speed of stream = y km/hr

⇒ SD = x + y

⇒ SU = x – y

TD = D/SD

⇒ TD = D/(x + y)

⇒ 4 = 20/(x + y)

⇒ x + y = 5      ---- (1)

TU = D/SU

⇒ TU = 20/(x – y)

⇒ 8 = 20/(x – y)

⇒ x – y = 2.5      ---- (2)

Solving (1) and (2)

⇒ x = 3.75 km/hr and y = 1.25 km/hr

∴ Speed of stream is 1.25 km/hr

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