To find the percentage tip, we can start by subtracting the cost of the dinner from the total amount Madelyn paid, which includes the tip.
Total amount paid (including tip) - Cost of dinner = Tip amount
$38.40 - $30.00 = $8.40 (Tip amount)
Now, we can calculate the percentage tip by dividing the tip amount by the cost of the dinner and multiplying by 100.
(Tip amount / Cost of dinner) x 100
($8.40 / $30.00) x 100 = 0.28 x 100 = 28%
Therefore, the percentage tip Madelyn left at the restaurant is 28%.