This question's explanation and your answers so far.
問90
頻出度★★★
電化製品を製造している G 社では，新たに開発した製品 H の発売を予定しており，その製造方法及び販売計画について検討を行っている。
製品 H の製造方法として案 A と案 B の二つの案が検討されていて，それぞれの案の固定費と変動費は表1のようになると見込んでいる。
表1 案Aと案Bの固定費と変動費の見込額
This is a graph in which the dashed line is parallel to the solid line (the same slope) and only the intercept has risen. That represents a case where the variable cost per unit is unchanged and only the fixed cost has increased — for example, when factory rent or equipment depreciation goes up. A rise in the purchase price of part J takes effect in proportion to the number of units, so this graph, whose slope does not change, is wrong.
×Incorrectイ
This is a graph in which the slope becomes larger and, at the same time, the intercept rises above 2,000. A higher intercept means the fixed cost has increased, but what increases when part J becomes more expensive is only the variable cost; the fixed cost of 20 million yen per month does not change. It depicts a situation where both the variable cost and the fixed cost have increased, so it is wrong.
○ウ
正解Manufacturing cost is expressed by the linear formula “fixed cost + variable cost per unit × number of units manufactured”. On the graph, the fixed cost is the vertical-axis intercept and the variable cost per unit is the slope. For plan B, the fixed cost is 20 million yen per month (shown as 2,000 on the graph, whose unit is 10,000 yen) and the variable cost is 50,000 yen per unit (Table 1). Part J is purchased from outside in order to be built into product H, so it is classified as a variable cost that arises each time one unit is made. Therefore, when part J becomes more expensive, only the variable cost per unit rises above 50,000 yen per unit, and the fixed cost of 20 million yen per month is not affected. As a result, the dashed line starts from the same intercept of 2,000 as the solid line and is a straight line with only a larger slope. The graph that shows this correctly is ウ.
×Incorrectエ
The intercept of 2,000 is correct, but the dashed line is drawn as a non-linear curve that bends sharply upward. The variable cost is a fixed amount per unit, so the cost is proportional to the number of units and the graph must be a straight line. A drawing in which the part cost per unit swells at an accelerating rate as the number of units increases does not match the premise, so it is wrong.
ここがポイント
Manufacturing cost = fixed cost + variable cost per unit × number of units manufactured. On the graph, the fixed cost = the vertical-axis intercept and the variable cost per unit = the slope. Judging which of the two changes is the heart of this question.
Costs that arise in proportion to the production volume, such as material costs and the cost of purchased parts, are variable costs; costs that arise as a fixed amount regardless of the production volume, such as rent and depreciation, are fixed costs. A rise in the purchase price of a part appears as an increase in the variable cost per unit, that is, an increase in the slope.