Given that,
Mass of a girl, m = 50 kg
Mass of the boat, m' = 300 kg
Speed of girl, v = 3 m/s
To find,
The velocity with which the boat begin to move backwards.
Solution,
Let the velocity be v'. Using the conservation of momentum to find it. It is given by :

So, the velocity moves backward with a speed of 0.5 m/s.