Input: To calculate the percentage change from 120 to 100, we need to follow the below steps:

Step 1: Calculate the difference between the two numbers. In this case, the difference is 20 (120-100).

Step 2: Divide the difference by the original number (120) and multiply by 100 to get the percentage change.

Percentage change = (20/120) x 100 = 16.67%

Step 3: Determine the direction of the change. Since the output (100) is less than the input (120), the percentage change is a decrease.

Therefore, the percentage change from 120 to 100 is a decrease of 16.67%.