You need to add 80% of $20 to $20.
First we calculate 80% of $20.
80% of $20 =
= 80% * $20
= 0.80 * $20
= $16
80% of $20 is $16.
That is the amount the store adds to the cost. This is called the markup.
Now we add the cost $20 and the markup $16.
$20 + $16 = $36
The selling price is $36
