📚 IGCSE CCEA Economics: Calculation Drill | IGCSE CCEA 经济：计算题专项训练
Mastering calculations is essential for success in IGCSE CCEA Economics. This drill covers the most common quantitative skills tested in the exam, from elasticities to national income accounting. Each section provides formulas, worked examples, and typical pitfalls to avoid.
掌握计算是取得 IGCSE CCEA 经济考试成功的关键。本专项训练涵盖了考试中最常见的定量技能，从弹性到国民收入核算。每个部分都提供公式、带解题步骤的示例以及需要避免的常见错误。
1. Percentage Changes | 百分比变化
Many economic calculations start with percentage changes. The formula is:
许多经济计算都始于百分比变化。公式如下：
% Change = (New Value − Original Value) ÷ Original Value × 100
Example: The price of a good rises from £5 to £6. The percentage change is ((6 − 5) ÷ 5) × 100 = 20%.
示例：某商品价格从 5 英镑上涨到 6 英镑。百分比变化为 ((6 − 5) ÷ 5) × 100 = 20%。
When values fall, the result is negative. Be careful to use the original value as the denominator.
当数值下降时，结果为负。注意要使用原始值作为分母。
2. Price Elasticity of Demand (PED) | 需求价格弹性
PED measures the responsiveness of quantity demanded to a change in price. The basic formula:
PED 衡量需求量对价格变动的反应程度。基本公式：
PED = %Δ Quantity Demanded ÷ %Δ Price
Always use absolute value and ignore the negative sign when interpreting. If |PED| > 1, demand is price elastic; if |PED| < 1, demand is price inelastic; if |PED| = 1, demand is unit elastic.
解释时始终使用绝对值并忽略负号。如果 |PED| > 1，需求富有弹性；如果 |PED| < 1，需求缺乏弹性；如果 |PED| = 1，需求为单位弹性。
Example: A cinema reduces ticket price from £10 to £8, and ticket sales increase from 100 to 150. %ΔP = (−2 ÷ 10)×100 = −20%. %ΔQd = (50 ÷ 100)×100 = 50%. PED = 50% ÷ 20% = 2.5 (absolute value). Demand is elastic.
示例：一家影院将票价从 10 英镑降到 8 英镑，售票量从 100 张增至 150 张。%ΔP = (−2 ÷ 10)×100 = −20%。%ΔQd = (50 ÷ 100)×100 = 50%。PED = 50% ÷ 20% = 2.5（绝对值）。需求富有弹性。
3. Price Elasticity of Supply (PES) | 供给价格弹性
PES indicates how quantity supplied responds to a price change. Formula:
PES 表示供给量对价格变动的反应程度。公式：
PES = %Δ Quantity Supplied ÷ %Δ Price
Unlike PED, PES is usually positive. A value greater than 1 means supply is elastic; less than 1, inelastic; equal to 1, unit elastic.
与 PED 不同，PES 通常为正值。大于 1 说明供给富有弹性；小于 1 缺乏弹性；等于 1 为单位弹性。
Example: The price of wheat rises from £200 to £220 per tonne, and farmers increase supply from 1000 to 1150 tonnes. %ΔP = 10%, %ΔQs = 15%, so PES = 1.5, elastic.
示例：小麦价格从每吨 200 英镑涨到 220 英镑，农民供给量从 1000 吨增加到 1150 吨。%ΔP = 10%，%ΔQs = 15%，因此 PES = 1.5，富有弹性。
4. Income Elasticity of Demand (YED) | 收入需求弹性
YED shows how demand changes as consumer income changes. Formula:
YED 显示需求随消费者收入变化而改变的程度。公式：
YED = %Δ Quantity Demanded ÷ %Δ Income
A positive YED means a normal good (luxuries have YED > 1, necessities 0 < YED < 1). A negative YED signifies an inferior good.
YED 为正意味着正常品（奢侈品 YED > 1，必需品 0 < YED < 1）。YED 为负表示低档品。
Example: Income rises by 15% and demand for organic vegetables rises by 20%. YED = 20% ÷ 15% = 1.33, a luxury good. Demand for bus travel falls by 5%, so YED = −5% ÷ 15% = −0.33, an inferior good.
示例：收入上升 15%，有机蔬菜需求上升 20%。YED = 20% ÷ 15% = 1.33，奢侈品。公交车出行需求下降 5%，则 YED = −5% ÷ 15% = −0.33，低档品。
5. Cross Elasticity of Demand (XED) | 交叉需求弹性
XED measures the responsiveness of demand for one good when the price of another good changes. Formula:
XED 衡量一种商品的需求对另一种商品价格变化的反应程度。公式：
XED = %Δ Quantity Demanded of Good A ÷ %Δ Price of Good B
A positive XED indicates substitutes; a negative XED indicates complements. The magnitude shows the strength of the relationship.
XED 为正表示替代品；XED 为负表示互补品。绝对值大小反映关系强弱。
Example: The price of tea increases by 10%, and demand for coffee rises by 4%. XED = 4% ÷ 10% = +0.4, substitutes but weak. If the price of printers falls by 5% and ink cartridge demand rises by 8%, XED = 8% ÷ −5% = −1.6, strong complements.
示例：茶的价格上涨 10%，咖啡需求上升 4%。XED = 4% ÷ 10% = +0.4，替代品但关系较弱。如果打印机价格下降 5%，墨盒需求上升 8%，XED = 8% ÷ −5% = −1.6，强互补品。
6. Total Revenue & Profit | 总收入和利润
Total revenue (TR) = Price × Quantity sold. Profit = TR − Total Costs (TC).
总收入 = 价格 × 销量。利润 = 总收入 − 总成本。
Example: A firm sells 500 units at £30 each. TR = 500 × £30 = £15,000. If TC is £12,000, Profit = £15,000 − £12,000 = £3,000.
示例：一家企业以每件 30 英镑的价格销售 500 件。总收入 = 500 × 30 = 15000 英镑。若总成本为 12000 英镑，则利润 = 15000 − 12000 = 3000 英镑。
Remember: if TR < TC, the firm makes a loss.
记住：如果总收入 < 总成本，企业亏损。
7. Costs of Production | 生产成本
Key cost concepts: Total Fixed Cost (TFC) stays constant; Total Variable Cost (TVC) changes with output; Total Cost (TC) = TFC + TVC. Average Cost (ATC) = TC ÷ Quantity. Marginal Cost (MC) = Change in TC ÷ Change in Quantity.
关键成本概念：总固定成本保持不变；总可变成本随产量变化；总成本 = 总固定成本 + 总可变成本。平均成本 = 总成本 ÷ 产量。边际成本 = 总成本变化量 ÷ 产量变化量。
Example: TFC = £1000. TVC for 100 units is £500. TC = £1500. ATC = £1500 ÷ 100 = £15. If producing 101 units raises TC to £1515, MC = £15.
示例：总固定成本 = 1000 英镑。生产 100 件产品的总可变成本为 500 英镑。总成本 = 1500 英镑。平均成本 = 1500 ÷ 100 = 15 英镑。若生产 101 件的总成本上升为 1515 英镑，则边际成本 = 15 英镑。
Be careful to distinguish between average and marginal cost.
注意区分平均成本与边际成本。
8. Break-even Analysis | 盈亏平衡分析
Break-even output occurs where TR
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