The original price of a watch was $550. during a mid year sale, the selling price was $440. find the percent discount

First, you would need to determine the discount amount. 
$550 - $440 = $110

Next, you would need to create an equation for this problem. Let x represent the unknown percent discount.
=

Cross multiply.
550x = 11,000

Now, all you would need to do is isolate the x. To do this, you would divide both sides of the equal sign by 550. 
x = 20
x = 20%

The percent discount was 20%. 

I hope this helps!