Consider the following table of arrival time and burst time for three processes P0, P1 and P2.

                                   Process                  Arrival time                    Burst time

                                       P0                             0 ms                                  9 ms

                                       P1                             1 ms                                  4 ms

                                       P2                             2ms                                   9 ms

The pre-emptive shortest job first scheduling algorithm is used. Scheduling is carried out only at arrival or completion of processes. What is the average waiting time for the three processes?