25% is the same as one quarter. Dividing 10 by 4 gives you 2.5.
If your restaurant bill comes to $10, a 25% tip would be $2.5, making your total $12.5. On a 25% off sale, an item originally priced at $10 would be discounted by $2.5, bringing the price to $7.5.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.25.