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Q.A man swimming down stream overcomes a float at a point M.After travelling distance...

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user8235304 | Student, Grade 11 | Valedictorian

Posted June 23, 2013 at 2:23 AM via web

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Q.A man swimming down stream overcomes a float at a point M.After travelling distance D,he turned back and passed the float at a distance of D/2 from the point M,then the ratio  of speed of swimmer with respect to still water to the speed of river is:

A)2

B)3

C)4

D)2.5

 

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llltkl | College Teacher | Valedictorian

Posted June 23, 2013 at 2:52 AM (Answer #1)

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The float is swimming along with the stream with speed say, r

While the man swimming downstream has a speed of (s+r), where, s is the speed of his swimming in still water, and r the speed of the river stream.

After reaching point M, he travels a distance D, then turns back and passes the float at a distance of D/2 from the point M.

Therefore, swimming downstream, he travels distance D with speed (s+r), then swims upstream till distance D/2 with a speed of (s-r). By that time the float has travelled distance D/2 with a speed of r.

Total time taken by the man in his swim to and fro =D/(s+r) + D/2/(s-r)

And by the float = D/2/r

By condition,

`D/(s+r) + D/(2(s-r)) = D/2r`

`rArr 1/(s+r) + 1/(2(s-r)) = 1/2r`

`rArr (2s-2r+s+r)/(2(s+r)(s-r)) = 1/2r`

`rArr 2r(3s-r)= 2 (s+r)(s-r)`

`rArr 3sr-r^2 = s^2-r^2`

`rArr 3r=s`

`rArr s/r =3`

So, the ratio of speed of the man in still water to the speed of the river stream was 3.

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