Waiting time of a process = finish time of that process - execution time - arrival time

Job 	Arrival 	Time 		Priority
A	0		12		1
B	2		4		3
C	5		2		1
D	8		10		3
E	10		6		2

A
1 2 3 4 5 6 7 8 9 10 11 12 (12) 

C
13 14 (2)

E
15 16 17 18 19 20 (6)

B
21 22 23 24 (4)

D
25 26 27 28 29 30 31 32 33 34 (10)


A - finished 12 - executed 12 - 0 arrival = 0 wait time
C - finished 14 - executed 2 - 5 arrival = 7 wait time
E - finished 20 - executed 6 - 10 arrival = 4 wait time
B - finished 24 - executed 4 - 2 arrival = 18 wait time
D - finished 34 - executed 10 - 8 arrival = 16 wait time

0 + 7 + 4 + 18 + 16 = 45/5 = 9