This question's explanation and your answers so far.
製品Aを1個生産するのに部品aが2個，部品bが1個必要である。部品aは1回の発注数量150個，調達期間1週間，部品bは1回の発注数量100個，調達期間2週間の購買部品である。製品Aの6週間の生産計画と，部品a，部品bの1週目の手持在庫が表のとおりであるとき，遅くとも何週目に部品を発注する必要があるか。ここで，部品の発注，納品はそれぞれ週の初めに行われるものとし，納品された部品はすぐに生産に利用できるものとする。
| 週 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| 製品Aの生産個数 | 0 | 40 | 40 | 40 | 40 | 40 |
| 部品a 所要数量 | 0 | 80 | 80 | 80 | 80 | 80 |
| 部品a 手持在庫数量 | 250 | |||||
| 部品a 発注数量 | ||||||
| 部品b 所要数量 | 0 | |||||
| 部品b 手持在庫数量 | 150 | |||||
| 部品b 発注数量 |
注記 網掛けの部分（空欄のセル）は，表示していない。
Week 2 is earlier than necessary. Part b runs short in week 5, and with a 2-week lead time, ordering in week 3 is enough to make it in time for the beginning of week 5. Not ordering in week 2 causes no delay, so it is not the latest week by which you must order “at the latest”.
正解 Using the MRP (Material Requirements Planning) approach, first find the week in which each part’s on-hand inventory falls short, then trace back from that week by the procurement lead time to determine the order week. Part a’s required quantity is twice that of Product A, namely 0, 80, 80, 80, 80, 80; subtracting these in turn from the on-hand inventory of 250 gives 170 in week 2, 90 in week 3, 10 in week 4, and in week 5, 10 − 80 = −70, so a shortage occurs in week 5. Since the lead time is 1 week, ordering in week 4 is enough to have it delivered at the beginning of week 5. Part b’s required quantity equals that of Product A (1 times), namely 0, 40, 40, 40, 40, 40; subtracting from the on-hand inventory of 150 gives 110 in week 2, 70 in week 3, 30 in week 4, and in week 5, 30 − 40 = −10, so it too runs short in week 5. Because Part b’s lead time is 2 weeks, it must be ordered in week 3 to be delivered at the beginning of week 5. Part a’s deadline is week 4 and Part b’s is week 3, and the earlier one, “at the latest week 3”, is the order deadline that lets both parts arrive in time. Therefore the correct answer is イ (week 3).
Week 4 is the deadline if you look only at Part a (1-week lead time). Part b has a 2-week lead time and runs short in week 5, so ordering in week 4 would mean delivery in week 6, one week late. It is too late as the order deadline for getting both parts in time.
If you order in week 5, Part a would be delivered in week 6 and Part b in week 7, and neither makes it in time for week 5 when the shortage occurs. Ordering is too late and production would stop, so this is wrong.