Average wait time for pre-emptive priority scheduling

Given the following table for calculating and average waiting time for the processes for priority based preemptive scheduling.

Process     Burst Time       Priority          
P1             10               3  
P2             1                1  
P3             2                4
P4             1                5
P5             5                2

The gantt chart would be as follows:

| P2 | P5 | P1 | P3 | P4  |
0    1    6    16   18   19

I have following questions:

1) Is the turn around time = 19 units?

2) How do i calculate average waiting time? Is there a formula?

3) What if few processes have the same priority?

I am new to OS. I have viewed some other similar questions but I did not get exactly how to do it.