Let velocity of the man = v metres/min.
Since he travels 100 m in 4 min, therefore
v = 100/4 m/min
= 25 m/min
In the flowing river, he takes 5 minutes.
⇒ He can travels 125 metres in 5 minutes with the speed of 25 m/min
Hence, during this time of 5 minutes, the river has flown 75 metres, i.e. speed of flowing water 15 m/min.