📚 A-Level WJEC Economics: Calculation Practice | A-Level WJEC 经济：计算题专项训练
This comprehensive guide covers the essential quantitative skills required for the WJEC A-Level Economics examination. Each section provides the formula, a worked example and the economic interpretation, enabling you to tackle calculation questions with confidence.
本综合指南涵盖了 WJEC A-Level 经济学考试所需的核心量化技能。每一节均提供公式、计算示例及经济学解释，帮助您自信应对计算题。
1. Price Elasticity of Demand (PED) | 需求价格弹性 (PED)
ures the responsiveness of quantity demanded to a change in price. The formula is: PED = (% Change in Quantity Demanded) ÷ (% e is usually negative, but we often ignore the minus sign and refer to its absolute value.
PED 衡量需求量对价格变化的反应程度。公式为：PED = (需求量变动的百分比) ÷ (价格变动的百分比)。请注意数值通常为负，但通常忽略负号，使用其绝对值。
Worked example: The price of a chocolate bar rises from £1.00 to £1.20. Quantity demanded falls from 500 units to 400 units. Calculate PED.
计算示例：巧克力棒价格从 1.00 英镑上涨至 1.20 英镑。需求量从 500 单位下降至 400 单位。计算 PED。
% Change in Price = (0.20 ÷ 1.00) × 100 = 20%. % Change in Quantity Demanded = (−100 ÷ 500) × 100 = −20%. PED = −20% ÷ 20% = −1. So |PED| = 1, indicating unit elastic demand.
价格变动百分比 = (0.20 ÷ 1.00) × 100 = 20%。需求量变动百分比 = (−100 ÷ 500) × 100 = −20%。PED = −20% ÷ 20% = −1。因此 |PED| = 1，表示单位弹性需求。
If |PED| > 1, demand is elastic (revenue rises when price falls). If |PED| < 1, demand is inelastic (revenue falls when price falls). This is crucial for firms' pricing strategies.
若 |PED| > 1，需求富有弹性（价格下降时收入上升）。若 |PED| < 1，需求缺乏弹性（价格下降时收入下降）。这对企业定价策略至关重要。
2. Price Elasticity of Supply (PES) | 供给价格弹性 (PES)
ES measures the responsiveness of quantity supplied to a change in price. Formula: PES = (% Change in Quantity Supplied) ÷ (% Change in Price). PES is positive because higher prices incentivise greater supply.
PES 衡量供给量对价格变化的反应程度。公式：PES = (供给量变动的百分比) ÷ (价格变动的百分比)。PES 为正数，因为价格上升激励供给增加。
Example: The price of wheat increases from £200 per tonne to £220 per tonne, and farmers increase supply from 1000 tonnes to 1100 tonnes. Calculate PES.
示例：小麦价格从每吨 200 英镑上涨至 220 英镑，农民将供给量从 1000 吨增加至 1100 吨。计算 PES。
% Change in Price = (20 ÷ 200) × 100 = 10%. % Change in Quantity Supplied = (100 ÷ 1000) × 100 = 10%. PES = 10% ÷ 10% = 1. This is unitary elastic supply.
价格变动百分比 = (20 ÷ 200) × 100 = 10%。供给量变动百分比 = (100 ÷ 1000) × 100 = 10%。PES = 10% ÷ 10% = 1。这是单位弹性供给。
Factors affecting PES include time period (supply is more elastic in the long run), spare capacity and ease of storage.
影响 PES 的因素包括时间周期（长期供给更具弹性）、闲置产能和储存难易度。
3. Income Elasticity of Demand (YED) | 需求收入弹性 (YED)
YED measures the responsiveness of demand to a change in income. Formula: YED = (% Change in Quantity Demanded) ÷ (% Change in Income).
YED 衡量需求对收入变化的反应程度。公式：YED = (需求量变动的百分比) ÷ (收入变动的百分比)。
Example: Consumer income rises from £30,000 to £33,000, and the quantity demanded for branded coffee increases from 200 cups to 230 cups per week. Calculate YED.
示例：消费者收入从 30,000 英镑上升至 33,000 英镑，品牌咖啡每周需求量从 200 杯增加至 230 杯。计算 YED。
% Change in Income = (3000 ÷ 30000) × 100 = 10%. % Change in QD = (30 ÷ 200) × 100 = 15%. YED = 15% ÷ 10% = +1.5. Since YED > 0 and > 1, this is a luxury good (income elastic).
收入变动百分比 = (3000 ÷ 30000) × 100 = 10%。需求量变动百分比 = (30 ÷ 200) × 100 = 15%。YED = 15% ÷ 10% = +1.5。由于 YED > 0 且 > 1，这是奢侈品（富有收入弹性）。
YED helps firms forecast sales during economic booms and recessions. Normal goods have YED > 0, inferior goods have YED < 0.
YED 有助于企业在经济繁荣和衰退时预测销量。正常品的 YED > 0，低档品的 YED < 0。
4. Cross Elasticity of Demand (XED) | 需求交叉弹性 (XED)
ness of demand for one good (A) to a change in price of another good (B). Formula: XED = (% Change in QD of A) ÷ (% Change in Price of B).
XED 衡量一种商品 A 的需求量对另一种商品 B 价格变化的反应程度。公式：XED = (A 需求量变动的百分比) ÷ (B 价格变动的百分比)。
Example: The price of cinema tickets rises by 8%, and the quantity demanded for popcorn falls by 12%. Calculate XED.
示例：电影票价格上涨 8%，爆米花需求量下降 12%。计算 XED。
XED = −12% ÷ 8% tive, the goods are substitutes. The magnitude indicates the strength of the relationship.
XED = −12% ÷ 8% = −1.5。负号表明电影票和爆米花是互补品。若 XED 为正，则为替代品。数值大小表明关联程度。
5. Costs of Production | 生产成本
C). Average Cost (AC) = TC ÷ Output. Marginal Cost (MC) = Change in TC ÷ Change in Output.
企业面临多种成本概念。总成本 (TC) = 总固定成本 (TFC) + 总可变成本 (TVC)。平均成本 (AC) = TC ÷ 产量。边际成本 (MC) = TC 的变动量 ÷ 产量的变动量。
Example: Output is 10 units, TFC = £200, TVC = £100. TC = £300, AC = £30. If increasing output to 11 units raises TC to £312, then MC = (312 − 300) ÷ (11 − 10) = £12.
示例：产量为 10 单位，TFC = 200 英镑，TVC = 100 英镑。TC = 300 英镑，AC = 30 英镑。若将产量增至 11 单位使得 TC 升至 312 英镑，则 MC = (312 − 300) ÷ (11 − 10) = 12 英镑。
In the short run, at least one factor is fixed. As output increases, initially MC falls due to specialisation, but eventually rises because of diminishing marginal returns. The AC curve is U-shaped.
短期内，至少一种生产要素固定。随产量增加，MC 最初因专业化而下降，但最终因边际报酬递减而上升。AC 曲线呈 U 形。
6. Revenue Concepts | 收入概念
Average Revenue (AR) = TR ÷ Q = Price. Marginal Revenue (MR) = Change in TR ÷ Change in Q.
总收入 (TR) = 价格 × 数量。平均收入 (AR) = TR ÷ 数量 = 价格。边际收入 (MR) = TR 的变动量 ÷ 数量的变动量。
Example: A firm sells 50 units at £10 each, TR = £500. To sell 51 units, it must lower price to £9.80. New TR = 51 × 9.80 = £499.80. MR of the 51st unit = 499.80 − 500 = −£0.20. This negative MR occurs because the price cut applies to all units.
示例：企业以每单位 10 英镑售出 50 单位，TR = 500 英镑。为售出第 51 单位，需降价至 9.80 英镑。新 TR = 51 × 9.80 = 499.80 英镑。第 51 单位的 MR = 499.80 − 500 = −0.20 英镑。出现负的 MR 是因降价适用于所有单位。
rice. Under imperfect competition, the firm faces a downward-sloping demand curve, so MR < AR.
在完全竞争下，AR = MR = 价格。在不完全竞争下，企业面临向下倾斜的需求曲线，因此 MR < AR。
7. Profit Maximisation | 利润最大化
= TR − TC. It is important to calculate the actual profit at the profit-maximising output.
企业在边际成本 (MC) = 边际收入 (MR) 处实现利润最大化。利润 = TR − TC。计算利润最大化产量下的实际利润很重要。
Example: A firm's output is 20 units, price = £20 (so MR = £20), MC = £20. TC = £300. TR = 20 × 20 = £400. Profit = 400 − 300 = £100. (Check: at 19 units, MR > MC; at 21 units, MR < MC, so 20 units is optimal).
示例：企业产量 20 单位，价格 = 20 英镑（故 MR = 20 英镑），MC = 20 英镑。TC = 300 英镑。TR = 20 × 20 = 400 英镑。利润 = 400 − 300 = 100 英镑。（检查：19 单位时 MR > MC；21 单位时 MR < MC，故 20 单位为最优。）
In a diagram, the profit-maximising output is where MC intersects MR from below. Be sure to use data tables to confirm the profit-maximising output.
在图形中，利润最大化产量是 MC 从下方与 MR 相交的位置。务必利用数据表确认利润最大化产量。
8. The Multiplier Effect | 乘数效应
MPC) or 1 ÷ (MPS + MPT + MPM). Alternatively, k = Change in National Income ÷ Change in Injection.
乘数衡量因初始注入循环流量而导致的国民收入变动。公式：乘数 (k) = 1 ÷ (1 − MPC) 或 1 ÷ (MPS + MPT + MPM)。或者，k = 国民收入的变动量 ÷ 注入的变动量。
Example: The marginal propensity to consume (MPC) is 0.8. Calculate the multiplier. If the government increases spending by £500 million, what is the total increase in national income?
示例：边际消费倾向 (MPC) 为 0.8。计算乘数。若政府增加支出 5 亿英镑，国民收入的总增加量是多少？
k = 1 ÷ (1 − 0.8) = 1 ÷ 0.2 = 5. Total increase in national income = 5 × £500m = £2,500m. Note: The larger the MPC, the larger the multiplier.
k = 1 ÷ (1 − 0.8) = 1 ÷ 0.2 = 5。国民收入总增加量 = 5 × 5 亿英镑 = 25 亿英镑。注意：MPC 越大，乘数越大。
9. Index Numbers | 指数
number = (Value in current year ÷ Value in base year) × 100. The base year is always 100.
指数简化了跨时期比较。指数 = (当年数值 ÷ 基年数值) × 100。基年始终为 100。
Example: The price of a basket of goods in 2020 (base year) was £200. In 2023, the same basket costs £260. Calculate the 2023 price index.
示例：2020 年（基年）一篮子商品的价格为 200 英镑。2023 年，同一篮子商品价格为 260 英镑。计算 2023 年的价格指数。
Index = (260 ÷ 200) × 100 = 130. This means prices have increased by 30% Indices are used for consumer price index (CPI), stock markets, and real GDP calculations.
指数 = (260 ÷ 200) × 100 = 130。这意味着自基年以来价格上涨了 30%。指数用于消费者价格指数 (CPI)、股票市场和实际 GDP 计算。
10. Nominal vs Real Values | 名义与实际价值
nal GDP ÷ Price Index) × 100. Similarly, a real wage = (Nominal Wage ÷ Price Index) × 100.
实际价值根据通胀调整名义价值。实际 GDP = (名义 GDP ÷ 价格指数) × 100。类似地，实际工资 = (名义工资 ÷ 价格指数) × 100。
Example: In 2022, nominal GDP was £2,000 billion and the price index (base year 2020) was 110. Calculate real GDP in constant 2020 prices.
示例：2022 年，名义 GDP 为 2 万亿英镑，价格指数（基年 2020 年）为 110。以 2020 年不变价格计算实际 GDP。
Real GDP = (2000 billion ÷ 110) × 100 = £ shows the true volume of goods and services produced, removing the impact of price rises.
实际 GDP = (2 万亿 ÷ 110) × 100 = 1.8182 万亿英镑。这显示了所生产的商品和服务的真实数量，剔除了价格上涨的影响。
11. Unemployment Rate & Labour Force Participation | 失业率与劳动参与率
vely seeking work. The participation rate = (Labour force ÷ Working-age population) × 100.
失业率 = (失业人数 ÷ 劳动力人数) × 100。劳动力包括就业者和积极求职者。参与率 = (劳动力人数 ÷ 劳动年龄人口) × 100。
Example: A country has a working-age population of 50 million. 35 million are employed, 5 million are unemployed and looking for work. Calculate the unemployment rate and participation rate.
示例：某国劳动年龄人口为 5000 万。3500 万就业，500 万失业且求职。计算失业率和参与率。
Labour force = 35m + 5m = 40m. Unemployment rate = (5 ÷ 40) × 100 = 12.5%. Participation rate = (40 ÷ 50) × 100 = 80%.
劳动力 = 3500 万 + 500 万 = 4000 万。失业率 = (5 ÷ 40) × 100 = 12.5%。参与率 = (40 ÷ 50) × 100 = 80%。
12. Balance of Payments: Current Account | 国际收支：经常账户
− Value of imports of goods. A current account deficit must be financed by a surplus on the financial/capital account.
经常账户包括货物贸易、服务贸易、初次收入和二次收入。贸易差额 = 货物出口价值 − 货物进口价值。经常账户赤字必须由金融/资本账户顺差来融资。
Example: A country exports £200bn of goods, imports £250bn of goods, has net service exports of +£20bn, net primary income of −£5bn, and net secondary income of +£3bn. Calculate the current account balance.
示例：某国出口货物 2000 亿英镑，进口货物 2500 亿英镑，服务贸易净出口 +200 亿英镑，初次收入净额 −50 亿英镑，二次收入净额 +30 亿英镑。计算经常账户余额。
Balance on goods = 200 − 250 = −£50bn. Total current account = −50 + 20 − 5 + 3 = −£32bn (deficit).
货物差额 = 200 − 250 = -500 亿英镑。经常账户总额 = −500 + 200 − 50 + 30 = -320 亿英镑（赤字）。
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