An OS follows round robin scheduling with a time quantum of 4 ms. Assuming that the CPU is free now and there are 20 processes waiting in the ready queue, what is the maximum amount of time that a process waits before getting into the CPU?

Explanation

In round robin, every process in the ready queue is given at most one time quantum (4 ms) before the scheduler moves to the next process in line. The process that waits longest for its first turn is the one sitting at the very back of the queue, which must let every one of the other 19 processes ahead of it use up to a full 4 ms quantum each before it is finally dispatched. Maximum wait = (20 − 1) × 4 ms = 19 × 4 = 76 ms, option (ii); 80 ms would be the (incorrect) result of counting the process's own quantum as part of its own wait.