Another way to think about it: for every 100 units in 10, you take 21. That adds up to 2.1.
If your restaurant bill comes to $10, a 21% tip would be $2.1, making your total $12.1. On a 21% off sale, an item originally priced at $10 would be discounted by $2.1, bringing the price to $7.9.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.21.