A discount is a percentage that is subtracted from a number. For example, a 10% discount of something that costs $30 is $3.
(10% converted to a decimal is .10 and .10 x 30 is 3).
So an item that sells for $30 and is being offered at a 10% discount can be bought for $27. ($30 - $3).
Now here's a problem for you:
If that item that sells for $30 were offered at a 20% discount, how much would you have to pay for it?