To find the percentage change from 100 to 80, we need to first calculate the difference between the two numbers.

Step 1: Difference = Final Value - Initial Value

Difference = 80 - 100

Difference = -20

Step 2: Next, we need to find the absolute value of the difference. This is because percentage change is always calculated as a positive value.

Absolute Difference = |Difference|

Absolute Difference = |-20|

Absolute Difference = 20

Step 3: Now, we can calculate the percentage change using the formula:

Percentage Change = (Absolute Difference / Initial Value) x 100

Percentage Change = (20 / 100) x 100

Percentage Change = 20%

Therefore, the percentage change from 100 to 80 is -20%. This means that there was a decrease of 20% from 100 to 80.