A person can directly cross a river of width 100 m in 4 minutes when there is no current, but it takes 5 minutes when there is current. What is the velocity of the current?

Answer 1

Nuts to this problem. If one is moving across the river, the v=25m/s.
If a current is at 90 degrees to the direction across the river, the time to get across is still 4 min. Now they may be moving downstream at some rate, but that does not affect how long it takes to get across. So the velocity of the current can be anything, it does not affect time to get across.