Discount: FAIL!
Suppose a Thing (TM) costs $100. Now,you are offered a 20% discount. You expect to pay $80 for it, right?
That means you expect to pay, for X discount, 100-X % of the cost.
That's why this picture is full of FAIL.
BTW: Real picture from a shop in Mar del Plata, Argentina, a few days ago.
You mean the negative discount? What else would you expect from a sign posted in a language that has double negatives? (An attribute I've never been able to understand, but it at least seems consistent with this sign's poor math.)
There ain´t no such thing as a double negative in english? ;-)