Another way to think about it: for every 100 units in 10, you take 66. That adds up to 6.6.
On a 66% off sale, an item originally priced at $10 would be discounted by $6.6, bringing the price to $3.4.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.66.