Processing times (including setup times) and due dates for six jobs waiting to be processed at a work centre are given in the table. The average tardiness (in days) using shortest processing time rule is __________ (correct to two decimal places).
$\begin{array}{|c|c|c|} \hline \text{Job} & \text{Processing time (days)} & \text{Due date (days)} \\ \hline A & 3 & 8 \\ \hline B & 7 & 16 \\ \hline C & 4 & 4 \\ \hline D & 9 & 18 \\ \hline E & 5 & 17 \\ \hline F & 13 & 19 \\ \hline \end{array}$