20% equals one fifth. Dividing 10 by 5 gives you 2.
If your restaurant bill comes to $10, a 20% tip would be $2, making your total $12. On a 20% off sale, an item originally priced at $10 would be discounted by $2, bringing the price to $8.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.2.