To decrease the full amount, you must first find the amount you are subtracting. Say you have $10 and you want to take away 22% of your ten. You would first multiply 10 by .22 because 22% is considered out of 100%.
10 x .22 = 2.2.
You are now subtracting $2.2 from your $10.
$10 - $2.20 = $7.80
So if you say got 22% off of a $10 shirt, you would pay $7.80 because 22% is deducted. I hope that helps! :)