A horizontal disc rotating freely about a vertical axis makes 100 rpm. A small piece of wax of mass 10 g falls vertically on the disc and adheres to it at a distance of 9 cm from the axis if the number of revolution per minute is thereby reduced to 90. Calculate the moment of inertia of disc.

Explanation

To solve the problem, we will use the principle of conservation of angular momentum. The angular momentum before the wax adheres to the disc must equal the angular momentum after it adheres.

### Step-by-Step Solution:

1. **Identify Initial Conditions**:
- Initial angular velocity (\( \omega_1 \)) = 100 revolutions per minute (rpm).
- Convert this to radians per second:
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