1.

A place is 21 km away from the point of start. Vina take 10 hour to row to the place and come back. If the speed of Vina in still water is 5 kmph, the speed of stream is ……… kmph.1). 22). 33). 1.54). 0.5

Answer»

Distance in upstream = distance in DOWNSTREAM = 21 km

Journey time = 10 hour

Speed of boat = 5 kmph

Speed of upstream = (5 – x) kmph

Speed of downstream = (5 + x) kmph

Let assume, speed of STREAM as x kmph,

Journey time = Time in upstream + time in downstream

⇒ 10 = 21/(5 – x) + 21/(5 + x)

⇒ 10(25 – x2) = 21(5 + x) + 21(5 – x)

250 – 10x2 = 105 + 21x + 105 – 21x

⇒ 250 - 210 = 10x2

⇒ x2 = 40/10

∴ x = 2 kmph


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