What process do you use to find the percent change of a quantity?

First, we need to get the amount of change:
The amount of change can either be an increase or a decrease.
It can be calculated as follows:
amount of change = 

If the amount of change is positive, then it is an increase
If the amount of change is negative, then it is a decrease

Then, we need to convert this amount into a percentage:
Changing the amount into a percentage can simply be done by multiplying this amount by 100
This means that:
% of change = amount of change * 100

Combining the two steps:
% of change =  * 100

Examples:
The original price of a certain product was $10. It then became $12. Find the % of increase.
% of change =  = 20%
This means that the price increased by 20%

The temperature decreased from 25 degrees to 20 degrees. Find the percentage of decrease.
% of decrease =  = -20%
This means that the temperature decreased by 20%

Hope this helps :)