Another way to think about it: for every 100 units in 10, you take 31. That adds up to 3.1.
On a 31% off sale, an item originally priced at $10 would be discounted by $3.1, bringing the price to $6.9.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.31.