Final answer:
To find the time it takes for the stone to be caught back and the velocity at which the stone is thrown, utilize kinematic equations for vertical motion and acceleration due to gravity.
Explanation:
The stone thrown by the girl reaches a height of 20 m. To calculate how long it takes for the stone to be caught back, you can use the kinematic equation for vertical motion: h = (1/2)gt^2, where h is the height, g is the acceleration due to gravity, and t is the time. Additionally, the velocity at which she throws the stone can be found using the equation: v = gt, where v is the velocity.
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