This question's explanation and your answers so far.
1台のCPUと1台の出力装置で構成されているシステムで，表の三つのジョブを処理する。三つのジョブはシステムの動作開始時点ではいずれも処理可能状態になっている。CPUと出力装置のそれぞれにおいて，ジョブ1，ジョブ2，ジョブ3の順に処理する。CPUと出力装置は独立して動作するが，出力処理はそれぞれのジョブのCPU処理が終了してから実施可能になる。ジョブ3の出力が完了するのは，ジョブ1の処理開始時点から何秒後か。
| CPU時間 | 出力時間 | |
|---|---|---|
| ジョブ1 | 35秒 | 10秒 |
| ジョブ2 | 20秒 | 20秒 |
| ジョブ3 | 5秒 | 25秒 |
30 sec is just job 3's CPU time of 5 sec plus its output time of 25 sec, which is nothing more than how long job 3 takes on its own. In reality job 3 can use the CPU only after jobs 1 and 2 (from 55 sec onward), and furthermore the output device is blocked by job 2's output until 75 sec. The CPU processing alone already takes 35+20+5=60 sec in total, so it is impossible for job 3's output to be finished at 30 sec.
45 sec is the time at which job 1's 10-second output finishes after its 35 seconds of CPU processing, in other words the time job 1's output completes. What the question asks for is the time job 3's output completes, so this option has the wrong target.
正解 Drawing a timing chart is the quickest route. There is only one CPU, so it processes the jobs back to back: job 1 from 0 to 35 sec, job 2 from 35 to 55 sec, and job 3 from 55 to 60 sec. There is also only one output device, and on top of that each job's output can begin only after that job's own CPU processing has finished. Job 1's output starts at 35 sec, right after its CPU processing ends, and runs for 10 sec, so 35 to 45 sec. Job 2's output cannot start until its CPU processing ends at 55 sec (the output device has been free since 45 sec, but it has to wait), so it runs from 55 to 75 sec. As for job 3, even though its CPU processing ends at 60 sec, the output device is still in use for job 2's output until 75 sec, so it is kept waiting; its output then runs for 25 sec from 75 sec, that is, 75 to 100 sec. Therefore job 3's output completes 100 seconds after the start of job 1's processing, and the correct answer is ウ. The key is to track on the time axis how the CPU and the output device run in parallel while, at the same time, waiting occurs because there is only one of each device.
115 sec is simply the total CPU time (35+20+5=60 sec) added to the total output time (10+20+25=55 sec); it is the answer you get if you wrongly assume that the CPU and the output device never operate in parallel, that is, that they run sequentially. The question states that the CPU and the output device operate independently, and in reality overlaps occur, such as job 2's CPU processing advancing while job 1's output is running, so the result is 100 sec, which is shorter than 115 sec.