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examBoard: Cambridge
examType: IGCSE
lessonTitle: Percentage and Ratio Applications
Geography isn't just about knowing where places are on a map - it involves understanding and analysing data to explain patterns and processes on Earth. Mathematical skills, especially percentages and ratios, are essential tools that help geographers make sense of complex information about populations, economies, environments and more.
Key Definitions:
Percentages help us understand proportions and make comparisons between different datasets. For example, comparing the percentage of urban population across different countries gives us a clearer picture than just using raw numbers.
Ratios allow us to express relationships between different values. For instance, the dependency ratio shows the relationship between the working-age population and those typically not in the labour force (children and elderly).
To calculate a percentage, we divide the value by the total and multiply by 100:
Percentage = (Value ÷ Total) × 100
For example, if a country has 30 million people living in urban areas out of a total population of 50 million:
Urban percentage = (30 ÷ 50) × 100 = 60%
Percentage change is crucial for understanding how geographical phenomena change over time. The formula is:
Percentage change = ((New value - Original value) ÷ Original value) × 100
A positive result indicates an increase, while a negative result indicates a decrease.
Calculate population growth rates, urbanisation percentages and demographic indicators like birth and death rates.
Analyse percentage changes in temperature, precipitation and sea levels to understand climate trends.
Compare GDP growth rates, employment sectors and trade balances as percentages to evaluate economic development.
China's population grew from 1.27 billion in 2000 to 1.41 billion in 2020. To calculate the percentage change:
Percentage change = ((1.41 - 1.27) ÷ 1.27) × 100 = 11.0%
This means China's population increased by 11.0% over this 20-year period. However, this growth rate is slowing down compared to previous decades, reflecting China's changing demographic policies and economic development.
A ratio compares quantities by division. Ratios can be written in several ways:
All these mean the same thing: for every 3 units of one thing, there is 1 unit of another.
Just like fractions, ratios should be expressed in their simplest form. To simplify a ratio, divide both numbers by their highest common factor.
For example, 15:25 can be simplified to 3:5 (dividing both by 5).
This important demographic indicator shows the number of dependents (aged 0-14 and over 65) compared to the working-age population (15-64).
Dependency ratio = (Number of dependents ÷ Working-age population) × 100
A higher ratio indicates more pressure on the productive population to support the economically dependent.
Map scales are ratios that show the relationship between distances on a map and actual distances on the ground.
For example, a scale of 1:50,000 means that 1cm on the map represents 50,000cm (or 500m) in real life.
Understanding scales is essential for measuring distances and areas on maps accurately.
Qatar has one of the world's most unbalanced gender ratios, with approximately 3.36 males for every female (a ratio of 3.36:1). This unusual ratio is due to the large number of male migrant workers in the construction and oil industries.
In contrast, Latvia has a ratio of 0.85:1, meaning there are more females than males. This imbalance is partly due to higher male mortality rates and emigration patterns.
These ratios have significant implications for social structures, marriage patterns and economic activities in these countries.
Geographers often use proportional symbols on maps to represent data visually. The size of the symbol is proportional to the value it represents.
For example, to represent city populations on a map:
This works because the area of a circle is proportional to the square of its radius (A = πr²). So if you want to represent a value that's 4 times larger, you need a radius that's 2 times larger (since 2² = 4).
Always show your calculations in the exam. Even if your final answer is incorrect, you can earn marks for using the right method.
Don't just calculate numbers - explain what they mean in geographical terms. What does a 15% increase in urban population suggest about a country's development?
Make sure you're using consistent units when calculating percentages and ratios. Converting everything to the same unit first will avoid errors.
In 2000, a city had a population of 450,000. By 2020, it had grown to 585,000.
Calculate the percentage change:
Percentage change = ((585,000 - 450,000) ÷ 450,000) × 100 = 30%
The city's population increased by 30% over the 20-year period.
A country has 120 million hectares of forest and 80 million hectares of agricultural land.
Express this as a ratio of forest to agricultural land:
Forest : Agricultural = 120 million : 80 million = 3 : 2
This means for every 3 hectares of forest, there are 2 hectares of agricultural land.
Mathematical skills involving percentages and ratios are essential tools for geographers. They allow us to:
By mastering these skills, you'll be better equipped to analyse geographical data and understand the complex patterns and processes that shape our world.
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