問14
頻出度★★☆
ある工場では, 部品 A, B から成る製品 P を組立生産している。部品 A の加工には, 部品 C を必要とし, 加工期間は 1 週間である。購買品である部品 B, C の納入リードタイムはそれぞれ 2 週間と 3 週間である。製品 P は部品 A, B がそろって初めて組み立てることができ, その組立生産に 1 週間掛かるとするとき, 製品 P を 1 個生産するために, 部品の納入期間も含めて最短で何週間必要か。ここで, すべての部品の在庫はないものとする。
平成22年度 秋期 ITパスポート試験 問14
This question's explanation and your answers so far.
解説
問 14
Incorrectア
4 weeks is a value that stops partway. C delivery 3 weeks + part A processing 1 week = 4 weeks is only the period “until part A is completed”, and the 1 week for assembling product P still remains after that. The question asks for the shortest period until one unit of product P is produced, so this is the mistake of forgetting to add the final 1 week of assembly production.
- イ
正解 The correct answer is イ (5 weeks). Since there is no stock, every part has to be counted starting from its delivery. The important point here is that purchasing part B and purchasing part C are independent of each other, so they can be carried out in parallel at the same time. Lining up the steps in time order gives the following. (1) Order part C → it is delivered 3 weeks later (week 0 → end of week 3). (2) Use the delivered C to process part A → this takes 1 week, so part A is completed at the end of week 4 (week 3 → end of week 4). (3) If part B is ordered at the same time, it has already been delivered by the end of week 2 and is simply waiting on hand. (4) Product P can only be assembled once both A and B are available, so assembly starts after waiting for the later one, the completion of part A (end of week 4); assembly production takes 1 week, so one unit of product P is completed at the end of week 5. The shortest required period is therefore 4 + 1 = 5 weeks. What determines the overall period is the longest path (the critical path), namely “C delivery 3 weeks → A processing 1 week → assembly 1 week”, which totals 5 weeks; the 2 weeks for B are hidden inside that path, so they do not affect the total duration.
Incorrectウ
6 weeks is the mistake of counting the purchases in series while also dropping the 1 week of processing for part A. If you assume that the 2 weeks of delivery for part B and the 3 weeks of delivery for part C are carried out one after the other, you get 2 + 3 = 5 weeks, and adding only the 1 week of assembly, without the 1 week of processing part A that is needed after C arrives, gives 6 weeks (counting A's processing as well in series gives 7 weeks, which is the path of エ). However, B and C are each independent purchased parts, so they can be ordered at the same time and their deliveries can be waited for in parallel. Stacking up work that can run in parallel as a series leads to overestimating the shortest period.
Incorrectエ
7 weeks is the mistake of adding up all the work in series. The calculation is C delivery 3 weeks + A processing 1 week + B delivery 2 weeks + assembly 1 week = 7 weeks, but the 2 weeks of delivery for part B can proceed in parallel with the delivery of C and the processing of A, so it does not need to be added separately as waiting time. In a problem that asks for the shortest period, the principle is to overlap work that can be done in parallel.
ここがポイント
- When calculating a required period, work that can proceed independently and at the same time should be run in parallel, and the overall period is determined by the longest path (the critical path). In this question the path “C delivery 3 weeks → A processing 1 week”= 4 weeks is longer than the path “B delivery 2 weeks”, so it is the limiting one, and adding 1 week of assembly gives 5 weeks.
- The basic way of thinking behind MRP (Material Requirements Planning). You expand the part structure of the product (P ← A + B, A ← C) and, using each part's procurement lead time and processing time, work backward from the planned completion date to decide when to place orders and start work. If stock is available, the procurement period can be shortened accordingly, but this question assumes there is no stock, so everything is counted from delivery.
用語