a person can swim in still water at speed 5 m/s . how much time will he take to move a distance of 120 m in upstream of water flowing at 1 m/s upstream and downstream respectively/

Going upstream means going against the water. Now the velocity of the swimmer with respect to the bank of the river will be 5-1=4 m/s. So the time taken to cross 120 distance will be 1204=30 s.
And while going downstream the velocity of swimmer with respect to bank is 5+1=6 m/s hence time taken is 1206=20 s.

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For upstream, v=5+1=6m/s
Time =distance /speed
Time =120/6=20sec
For downstream, v=5-1=4m/s
Time =distance /speed
Time =120/4=30m/s
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