This question's explanation and your answers so far.
機械メーカのS社では，製品Xに組み込むソフトウェア（以下，Xソフトという）の開発作業A～Hを表1のように計画した。ここで，前作業とは当該作業を開始する前に終了していなければならない作業のことであり，各作業は前作業が終了すればすぐに開始する。各作業に掛かった費用は，各作業完了時に計上する。 表1 Xソフト開発の作業
| 作業 | 日数 | 費用 | 前作業 |
|---|---|---|---|
| A | 2 | 5 | なし |
| B | 3 | 7 | A |
| C | 4 | 4 | B |
| D | 4 | 5 | A |
| E | 5 | 6 | D |
| F | 2 | 5 | A |
| G | 6 | 6 | D, F |
| H | 7 | 12 | C, E, G |
図1は，表1に基づいて作成したアローダイアグラムである。 Xソフトの開発が表1のとおりに進んだ場合，開始してから10日目の作業完了時までに計上する費用の累計を表すグラフとして，適切なものはどれか。
It also starts at 0, has steps on days 2, 4, 5 and 6, and ends at 26, so its course up to day 9 is the same as イ; however, the last step (+4, the cost of C) is placed on day 8. C takes 4 days after B (which finishes on day 5), so it is completed on day 9 and cannot be recorded on day 8. The step is one day too early, which makes this graph wrong.
正解 The cost of each activity is recorded at the time it is completed, so first find the completion day of each activity from the preceding-activity constraints. A has no preceding activity, so it starts on day 0 and finishes on day 2. Once A is finished, B, D and F can all start: F takes 2 days and finishes on day 4, B takes 3 days and finishes on day 5, and D takes 4 days and finishes on day 6. C starts after B finishes (day 5), takes 4 days and finishes on day 9. E starts after D finishes (day 6), takes 5 days and finishes on day 11; G can start only on day 6, when both D and F are finished, takes 6 days and finishes on day 12. Therefore only five activities — A, F, B, D and C — are completed by day 10, and the cumulative cost rises to 5 on day 2, 10 on day 4 (+5), 17 on day 5 (+7), 22 on day 6 (+5) and 26 on day 9 (+4); because E and G are not yet finished, it is still 26 on day 10. The graph starts at 0, has steps on days 2, 4, 5, 6 and 9, and stays flat at 26 from day 9 onward — only graph イ has this shape.
A cost of 5 is already recorded at day 0, which means the cost of A is recorded at its start instead of at its completion (day 2). In addition, the steps fall on days 2, 3, 4, 6 and 9, all shifted earlier, and the cumulative cost on day 10 is about 32, a value that already includes the costs of E and G. On day 10, E (finished on day 11) and G (finished on day 12) are not yet complete, so the total cannot reach that amount.
As in ウ, recording the cost of A on day 0 is wrong, and the steps fall on days 2, 3, 4, 7 and 9, which do not match the actual completion days (2, 4, 5, 6 and 9). The cumulative cost on day 10 is also about 32, an excessive value that includes the costs of E and G even though they are not yet finished.