整合原書 Figure 7-17～7-21：先理解 ROUND／ROUNDUP／ROUNDDOWN，再用 Call Center 五年模型示範 stepped cost。
原書 Call Center assumptions：每位 operator 可服務 40 customers；Year 1 customers 500，之後每年成長 20%；fixed costs $250,000；variable overhead $100/customer；operator cost $65,000/year；所有 costs 每年 inflation 3%。Operator 必須以整數配置，因此需要 ROUNDUP。
| Assumption | 原書值 |
|---|---|
| Customers per Operator | 40 |
| Year 1 Customers | 500 |
| Annual Customer Growth | 20% |
| Fixed Cost | $250,000 |
| Variable Overhead per Customer | $100 |
| Operator Cost per Year | $65,000 |
| Annual Cost Inflation | 3% |
Customers 每年以前一年 ×1.20 compounding。Operators = ROUNDUP(Customers/40,0)。Fixed cost、variable unit cost、operator unit cost 各自每年 ×1.03。Total Cost = Fixed + Variable + Operator Cost。
| Year | Customers | Operators | Fixed Cost | Variable Cost | Operator Cost | Total Cost |
|---|---|---|---|---|---|---|
| 1 | 500 | 13 | 250,000 | 50,000 | 845,000 | 1,145,000 |
| 2 | 600 | 15 | 257,500 | 61,800 | 1,004,250 | 1,323,550 |
| 3 | 720 | 18 | 265,225 | 76,384.8 | 1,241,253 | 1,582,862.8 |
| 4 | 864 | 22 | 273,181.75 | 94,411.61 | 1,562,599.61 | 1,930,192.97 |
| 5 | 1,036.8 | 26 | 281,377.20 | 116,692.75 | 1,902,109.89 | 2,300,179.84 ≈ 2,300,180 |
Operator cost 是 stepped cost：customers 增加並不會讓 headcount 線性平滑增加，而是在容量被突破時跳到下一位 operator。這就是 ROUNDUP 比一般 ROUND 更適合 staffing capacity 的原因。
請只根據原書 Call Center assumptions 與五年計算，解釋 fixed、variable、stepped cost 的不同，並說明為什麼 Year 1 的 500/40=12.5 必須 ROUNDUP 成 13 operators。
**問題：**如果 Year 1 customers 改成 480，其他條件不變，Year 1 operators 需要多少？