An invoice for a shipment is shown below. The delivery fee is calculated as a percentage of the cost of the order. The grand total is the sum of the cost of the order and the delivery fee.

$$\text{Invoice Details}$$ $$\begin{array} {c|c} 20 & \text{Item \#3892} \\ 15 & \text{Item \#2215} \\ 5 & \text{Item \#3100} \\ \end{array}$$ $$\begin{aligned} \text{Delivery Fee: }& \$25.28 \\ \text{Grand Total: }& \$193.78 \end{aligned}$$

What percentage of the cost of the order was used to calculate the delivery fee?

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We first calculate the order total.

$$\text{Grand Total}-\text{ Delivery Fee}=\text{Order Total}$$ $$193.78-25.28=168.50$$

The percentage of the delivery fee of the total can be calculated:

$$\% \text{ delivery fee} = \frac{25.28}{168.50}$$ $$=0.15 = \boxed{15 \%}$$