The following table shows the processes in the ready queue and time required…

Concept

Round Robin (RR) is a preemptive CPU-scheduling algorithm: every ready process is given the CPU for at most one fixed time quantum, after which it is preempted and sent to the tail of the ready queue if it still has work left. Processes are served in a circular (FIFO) fashion until each finishes.

Key definitions used below: Completion Time (CT) is the instant a process finishes; Waiting Time (WT) = CT - Burst Time - Arrival Time; the average waiting time is the mean of all the WT values.

Assumption on arrival time: when a problem gives no arrival times, the standard convention is that all processes arrive together at time 0 and enter the ready queue in the listed order (P1, P2, P3, P4, P5). With arrival time = 0 for every process, the waiting-time formula simplifies to WT = CT - Burst Time.

Step 1 - Build the Gantt chart (quantum = 5 ms)

Cycle through the queue, giving each process 5 ms (or its remaining time, whichever is smaller). A process that still has time left re-joins the back of the queue. Remaining bursts start at P1=10, P2=5, P3=20, P4=8, P5=15.

1. 0-5: P1 runs (10 -> 5 left, requeued)

2. 5-10: P2 runs (5 -> 0, P2 finishes, CT = 10)

3. 10-15: P3 runs (20 -> 15 left, requeued)

4. 15-20: P4 runs (8 -> 3 left, requeued)

5. 20-25: P5 runs (15 -> 10 left, requeued)

6. 25-30: P1 runs (5 -> 0, P1 finishes, CT = 30)

7. 30-35: P3 runs (15 -> 10 left, requeued)

8. 35-38: P4 runs (3 -> 0, P4 finishes, CT = 38)

9. 38-43: P5 runs (10 -> 5 left, requeued)

10. 43-48: P3 runs (10 -> 5 left, requeued)

11. 48-53: P5 runs (5 -> 0, P5 finishes, CT = 53)

12. 53-58: P3 runs (5 -> 0, P3 finishes, CT = 58)

Step 2 - Waiting Time = Completion Time - Burst Time

Step 3 - Average waiting time

Sum of waiting times = 20 + 5 + 38 + 30 + 38 = 131 ms.

Average waiting time = 131 / 5 = 26.2 ms.

Cross-check

The last process to finish (P3) completes at t = 58 ms, which equals the total of all burst times (10 + 5 + 20 + 8 + 15 = 58) - consistent because RR has no idle gaps when every process is present from t = 0. The average turnaround time is (30 + 10 + 58 + 38 + 53)/5 = 37.8 ms, and 37.8 - (58/5 average burst = 11.6) = 26.2 ms, matching the average waiting time.