b.Shortest Remaining Time: (20+0+70+10)/4 = 25 ms.Explanation: P2 does not wait, but P1 waits 20ms, P3 waits70ms and P4 waits 10ms.Non-preemptive Priority: (0+30+10+70)/4 = 27.5msExplanation: P1 does not wait, P2 waits 30ms until P1 finishes,P4 waits only 10ms since it arrived at 60ms and it is scheduled at70ms. P3 waits 70ms.Round-Robin: (20+10+70+70)/4 = 42.5msExplanation: P1 waits only for P2 (for 20ms). P2 waits only 10msuntil P1 finishes the quantum (it arrives at 20ms and the quantum is30ms). P3 waits 30ms to start, then 40ms for P4 to finish. P4 waits40ms to start and one quantum slice for P3 to finish.