Answer:
The answer is 10m/s
Step-by-step explanation:
Given that the child is in the train, and that the train is moving at a speed of 10m/s, and also the child is standing on the flatbed, hence the child is not moving, therefore the velocity of the child is 10m/s relative to the observer
Also, when the child throws the ball horizontally relative to the observer the horizontal speed is also 10m/s.
therefore the speed of the ball is also 10m/s to the observer