Speed in still water is 5 km/h
Let the velocity of the river be m km/h
Width of the river is 1 km and time taken is 15 minutes, that is 14 hours.
Using the formula, d = s×t
d = 5 × 14 = 1.25km
If the river had been still, the boat would have moved from A to B, 1 km but, the boat moves from A to C
⇒ CB2 = AC2−AB2
⇒ CB2 = 1.252−12
⇒ CB2 =0.5625
∴CB = 0.75
The covered distance by the river water in 15 minutes is 0.75km.
Now, the velocity of the river is
v = d/t = 0.75/1/4 =3km/h