Jaigant, the wording of your question leaves room for a couple of interpretations. If 80 bucks is the 'pre-discount' pricing, then the previous posts give you what you need.
On the other hand, you might be asking, "this item's on sale at $80, and this price represents a 10% discount off of its usual price, so what was the original price?"
If that's the case, then 80 is 90% of the original price
P. Or...
80 = 0.9
P
To find P, divide both sides by 0.9...
To wrap it up, just confirm for yourself that if you knock 10% off a price of $88.89 you'll end up with a sale price of $80. I'll leave that fun for you.