Answer:
Rotational inertia of the object is,

Step-by-step explanation:
Given that,
Mass of the object, m = 20 kg
Torsion constant of the wire, K = 0.85 N-m
Number of cycles, n = 69
Time, t = 66 s
To find,
The rotational inertia of the object.
Solution,
There exists a relationship between the moment of inertia, time period and the torsion constant of the spring is given by :

Here I is the moment of inertia
T is the time period, and it is equal to the number of cycles per unit time



So, the rotational inertia of the object is
.