Another way to think about it: for every 100 units in 10, you take 42. That adds up to 4.2.
On a 42% off sale, an item originally priced at $10 would be discounted by $4.2, bringing the price to $5.8.
Whether you are calculating a discount, a tax, a tip, or a commission, the math is always the same: multiply 10 by 0.42.