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Asked by adjacentcalliber10 | 18 May, 2019, 13:15: PM
answered-by-expert Expert Answer
Let v m/s be the swimming speed of man and u m/s be the speed of river.
 
for minimum time  t to cross the river of width d, we have  t = d / v  ..............(1)
 
By substituting the values in eqn.(1), we have  25 = 100 / v   or   v = 4 m/s
 
drift distance x for minimum time crossing is given by,  x = u×(d / v)  ....................(2)
 
By substituting the values in eqn.(2), we have  75 = u×(100/4)   or   u = 3 m/s
 
time taken t1 for 70 m downstream = 70 / (4+3) = 10 s
 
time taken t2 for 70 m upstream = 70 / (4-3) = 70 s
 
hence ( t2 / t1 )  = 70/10 = 7
Answered by Thiyagarajan K | 18 May, 2019, 17:04: PM

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