Another way to think about it: for every 100 units in 10, you take 51. That adds up to 5.1.
On a 51% off sale, an item originally priced at $10 would be discounted by $5.1, bringing the price to $4.9.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.51.