This question's explanation and your answers so far.
ある商品の4月の仕入と売上が表のとおりであるとき，移動平均法による4月末の商品の棚卸評価額は何円か。移動平均法とは，仕入の都度，在庫商品の平均単価を算出し，棚卸評価額の計算には直前の在庫商品の平均単価を用いる方法である。
| 日付 | 摘要 | 入庫数量（個） | 入庫単価（円） | 入庫合計（円） | 出庫数量（個） | 出庫単価（円） | 出庫合計（円） | 在庫数量（個） | 在庫平均単価（円） | 在庫合計（円） |
|---|---|---|---|---|---|---|---|---|---|---|
| 4月1日 | 繰越 | 100 | 10 | 1,000 | 100 | 10 | 1,000 | |||
| 4月8日 | 仕入 | 100 | 14 | 1,400 | 200 | 12 | 2,400 | |||
| 4月18日 | 売上 | 150 | 50 | |||||||
| 4月29日 | 仕入 | 50 | 16 | 800 | 100 |
注記 網掛けの部分は，表示していない。
This is the value calculated with the periodic (total) average method — which takes a single weighted average over the whole period — rather than the moving average method. (1,000+1,400+800) ÷ (100+100+50) = 3,200 ÷ 250 = 12.8 yen, and 100 units × 12.8 = 1,280 yen. Its premise differs from the moving average method, which updates the unit price on every purchase.
This is the mistake of valuing the 50 units remaining after the April 18 sale at the carried-forward unit price of 10 yen instead of the immediately preceding moving average unit price of 12 yen. (50×10 + 50×16) ÷ 100 = 13 yen, and 100 units × 13 = 1,300 yen. The remaining stock must always be carried over at the immediately preceding average unit price (12 yen).
正解 Under the moving average method, each time a purchase is made the average unit price is updated as (total inventory value) ÷ (inventory quantity), and that unit price is used for subsequent issues and valuation. On April 1 the carried-forward stock is 100 units with a total of 1,000 yen, giving an average unit price of 10 yen. On April 8, a purchase of 100 units × 14 yen = 1,400 yen is added, so the stock becomes 200 units with a total of 2,400 yen, and the average unit price is 2,400 ÷ 200 = 12 yen. The sale of 150 units on April 18 is issued at the immediately preceding average unit price of 12 yen, so the remaining stock is 200 − 150 = 50 units, valued at 50 × 12 = 600 yen. On April 29, a purchase of 50 units × 16 yen = 800 yen is added, so the stock becomes 100 units with a total of 600 + 800 = 1,400 yen, and the average unit price is 1,400 ÷ 100 = 14 yen. Therefore the inventory valuation amount at the end of April is 100 units × 14 yen = 1,400 yen, so ウ is correct.
This is the mistake of not weighting by quantity, but taking a simple average of the unit prices 14 yen and 16 yen to get 15 yen. 100 units × 15 = 1,500 yen. Because the moving average weights by quantity, the correct calculation is (600+800) ÷ 100 = 14 yen, from the remaining 50 units × 12 yen and the purchased 50 units × 16 yen.