Method1:FCFS(first-come, first-served)

process    duration

p1              18

p2              4

p3              20

p1 waits 0s. p2 waits 18s. p3 waits 22s.

the average waiting time is (0 + 18 + 22) / 3 = 13.333333...

Method2:SJF(shortest-job-first)

process    duration

p1              18

p2              4

p3              20

process execution sequence: p2, p1, p3

p2 waits 0s. p1 waits 4s. p3 waits 22s

the average waiting time is (0 + 4 + 22) / 3 = 8.6666...

Method3: priority scheduling algorithm

process    duration   priority

p1              18              3

p2              4               2

p3              20             1

process execution sequence: p3, p2, p1

p3 waits 0s, p2 waits 20s, p1 waits 24s.

the average waiting time is(0 + 20 + 24) / 3 = 14.6666666

Method4: RR(round-robin)

process    duration

p1              16

p2              4

p3              20

assume that the time slice is 4s

process execution sequence: p1, p2(end), p3, p1, p3, p1, p3, p1(end), p3, p3(end)

p2 waits 4s. p1 waits 60s, p3 waits 80s.

the average waiting time is(4 + 60 + 80) / 3 = 48(Isn't it too long?)