整合原書 Table 5.1–5.2，計算 Amazon 2020 Inventory Turnover Days 與 Days Receivable。
Inventory 應配對 COGS；Accounts Receivable 應配對 Total Net Sales。原書使用 2019／2020 average ending balances 計算 2020 days。
| US$ millions | 2019A | 2020A |
|---|---|---|
| Inventories | 20,497.0 | 23,795.0 |
| Accounts Receivable, net and other | 20,816.0 | 24,542.0 |
| 2020 COGS | — | 233,307.0 |
| 2020 Total Net Sales | — | 386,064.0 |
Inventory Days = AVERAGE(2019 Inventory, 2020 Inventory) ÷ 2020 COGS × 360；Receivable Days = AVERAGE(2019 A/R, 2020 A/R) ÷ 2020 Net Sales × 360。
| 指標 | 2020A |
|---|---|
| Inventory Turnover Days | 34.2 |
| Days Receivable | 21.1 |
| Total Operating Current Assets 2019 | 41,313.0 |
| Total Operating Current Assets 2020 | 48,337.0 |
34.2 Inventory Days 與 21.1 Receivable Days 將 Balance Sheet balances 轉成與 COGS／Revenue 規模可比較的營運速度指標。原書後續把 2020 days 當作 projection driver。
請根據 Amazon 2019A–2020A Inventory、A/R 與 2020 days 結果，說明為什麼 Inventory 要配 COGS，而 A/R 要配 Revenue。
**問題：**若 2020 Days Receivable 上升到 30 days，2020 Revenue 386,064 且用簡化公式 Days/360 × Revenue，A/R 約是多少？