This question's explanation and your answers so far.
製品Aの生産計画量，部品Bの総所要量及び在庫量が表のとおりであるとき，第2週における部品Bの発注量aは何個か。 〔条件〕 ・製品Aの生産リードタイム（着手から完成までの期間）は無視する。 ・製品Aを1個生産するためには部品Bが2個必要であり，部品Bは製品Aの生産以外には使われない。 ・部品Bの発注は，各週の生産終了後に行い，翌週の生産開始までに入荷する。 ・部品Bの安全在庫は，当該週の部品Bの総所要量の25%とする。 ・部品Bの第1週の生産開始前の在庫量を100個とする。 単位 個
| 第1週 | 第2週 | 第3週 | |
|---|---|---|---|
| 製品Aの生産計画量 | 40 | 40 | 20 |
| 部品Bの総所要量 | 80 | 80 | 40 |
| 部品Bの在庫量（生産終了後） | 20 | ||
| 部品Bの発注量 | a |
注記 網掛けの部分は表示していない。
正解 The order quantity a is placed after production ends in week 2 and arrives before production starts in week 3, so it covers the production of week 3. The inventory after production each week is kept at that week's safety stock (25% of the gross requirement); in fact the week-1 ending inventory of 20 matches 25% of the week-1 gross requirement of 80, which confirms this. Therefore the inventory after week 2 is 80×25%=20. The inventory required at the start of production in week 3 is the week-3 gross requirement of 40 plus the week-3 safety stock (40×25%=10), giving 50. Subtracting the 20 on hand, the order quantity a=50-20=30. Hence the correct answer is ア.
This wrongly takes the week-3 gross requirement of 40 directly as the order quantity. In fact you must subtract the inventory of 20 left on hand after production in week 2 and allow for the week-3 safety stock of 10 (40-20+10=30).
This neither subtracts the 20 left on hand nor uses the correct safety stock; it mistakenly treats the safety stock as 25% of the week-2 gross requirement of 80 (=20) and adds it to the week-3 gross requirement of 40, giving 40+20=60. Correctly, the safety stock is 10 based on week 3, and the on-hand inventory of 20 must be subtracted.
The order quantity a covers the production of the following week, namely week 3, yet this mistakenly takes the week-2 gross requirement of 80 (the very week in which the order is placed) as the order quantity. It confuses the timing of the order with the requirement it is meant to cover.