The average velocity is -4.17 m/s
Explanation:
The average velocity of a body is given by:

where
d is the displacement of the body
t is the time elapsed
For the student in this problem, we have:
Initial position: 
Final position: 
So the displacement is

The time elapsed is
t = 60 s
Therefore, the average velocity is

Where the negative sign means the student is moving towards the origin.
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The mass on the spring is 0.86 kg
Explanation:
The period of a mass-spring system is given by the equation

where
m is the mass
k is the spring constant
In this problem, we have:
k = 88.7 N/m is the spring constant
The system makes 15 oscillations in 9.24 s: therefore, the period of the system is

Now we can re-arrange the first equation to solve for the mass:

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