For FCFS
each processes will get 20ms CPU time first, hence total CPU time for all 3 processes = 60ms. Now first process will complete I/O within 100ms. After 100 ms the rest of the processes will start their CPU cycles again one after the other.
Hence in a period of 100ms total CPU time = 60ms
CPU utlization = 60%
For Round robin
the I/O burst of the first process starts after (10 + 10 + 10 + 10)ms = 40ms after it the other two processes will complete their CPU bursts in the next 20ms hence total CPU time = 60 ms.
The I/O burst of the first process will complete by 120 ms after which the processes will start their CPU cycles in round robin pattern again.
Hence in a period of 120ms , 60 ms of CPU time used.
hence CPU utilization = 50%
Difference = (FCFS pattern) – (Round robin pattern) = 10%