Answer:
Vf = 69.61 m/s
Explanation:
We will use the third equation of motion to solve this problem:

where,
g = acceleration due to gravity = 9.81 m/s²
h = height of cliff = 247 m
Vf = final velocity = ?
Vi = initial velocity = 0 m/s (boulder breaks loose from rest)
Therefore,

<u>Vf = 69.61 m/s</u>
Avg acceleration; under constant acceleration condition only; = Vf-Vi/time
a=Vf-Vi/time=(15m/s-6m/s)/(3s)=
(9m/s)/(3s)=3m/s^2
Hope this helps. Thanks.
The answer is 20. Frequency is "number of times" per unit time.
It's the number of protons that are in an element