Speed of bird = 25km / h, it means a bird covers a distance of 25 km in 1 hour or 60 minutes.
A birds covers a distance of 1 km in time = 1 hour = 60 min
so, 60 / 25 = 2.4 = 2 minutes 4 second
it means a bird covers a distance of 1 km in 2 minutes 4 second, so a bird take time to fly 3.5 km in time = 3.5 x 2.4 = 8.4 = 8 minutes 4 second
So, the answer is 8 minutes 4 seconds.