Understanding Percentages as Fractions and Decimals
Introduction
Percentages are fundamental concepts in mathematics, especially within the curriculum of IB MYP 1-3. Understanding how percentages relate to fractions and decimals is crucial for developing strong problem-solving skills and applying mathematical concepts to real-world scenarios. This article delves into the relationship between percentages, fractions, and decimals, providing a comprehensive guide for students studying ratios, proportions, and percentages.
Key Concepts
1. Definitions and Basic Understanding
Percentage is a way of expressing a number as a fraction of 100. It is denoted using the percent sign (%). For example, 45% means 45 out of 100.
Fraction represents a part of a whole and is expressed as one number over another, such as $\frac{3}{4}$.
Decimal is another way to represent fractions, using the base-10 system. For example, $\frac{3}{4}$ is equivalent to 0.75 in decimal form.
Understanding these three representations is essential as they are interchangeable and frequently used in various mathematical contexts.
2. Converting Percentages to Fractions
To convert a percentage to a fraction:
- Write the percentage as the numerator with 100 as the denominator.
- Simplify the fraction by dividing both the numerator and the denominator by their greatest common divisor (GCD).
Example: Convert 60% to a fraction.
$$
\frac{60}{100} = \frac{3}{5}
$$
3. Converting Fractions to Percentages
To convert a fraction to a percentage:
- Divide the numerator by the denominator to get a decimal.
- Multiply the decimal by 100 and add the percent sign.
Example: Convert $\frac{3}{4}$ to a percentage.
$$
\frac{3}{4} = 0.75 \\
0.75 \times 100 = 75\%
$$
4. Converting Percentages to Decimals
To convert a percentage to a decimal:
- Remove the percent sign.
- Divide by 100 or move the decimal point two places to the left.
Example: Convert 85% to a decimal.
$$
85\% = \frac{85}{100} = 0.85
$$
5. Converting Decimals to Percentages
To convert a decimal to a percentage:
- Multiply the decimal by 100.
- Add the percent sign.
Example: Convert 0.65 to a percentage.
$$
0.65 \times 100 = 65\%
$$
6. Converting Fractions to Decimals
To convert a fraction to a decimal:
- Divide the numerator by the denominator.
Example: Convert $\frac{7}{8}$ to a decimal.
$$
\frac{7}{8} = 0.875
$$
7. Finding a Percentage of a Number
To find a percentage of a number:
- Convert the percentage to a decimal.
- Multiply the decimal by the number.
Example: Find 20% of 150.
$$
20\% = 0.20 \\
0.20 \times 150 = 30
$$
8. Increasing or Decreasing a Number by a Percentage
To increase a number by a percentage:
$$
\text{New Value} = \text{Original Value} + (\text{Original Value} \times \text{Percentage})
$$
To decrease a number by a percentage:
$$
\text{New Value} = \text{Original Value} - (\text{Original Value} \times \text{Percentage})
$$
Example: Increase 200 by 15%.
$$
200 + (200 \times 0.15) = 200 + 30 = 230
$$
9. Percentage Change
Percentage change calculates the relative change between two numbers.
$$
\text{Percentage Change} = \left( \frac{\text{New Value} - \text{Original Value}}{\text{Original Value}} \right) \times 100\%
$$
Example: Original Value = 50, New Value = 75.
$$
\left( \frac{75 - 50}{50} \right) \times 100\% = 50\%
$$
10. Applications in Real Life
Percentages are widely used in various real-life contexts, such as:
- Finance: Calculating interest rates, discounts, and profit margins.
- Statistics: Representing data proportions in surveys and research.
- Health: Understanding nutritional content and body measurements.
- Education: Grading systems and performance metrics.
Understanding percentages as fractions and decimals enables students to apply mathematical concepts effectively in these areas.
11. Compound Percentages
Compound percentages involve applying multiple percentage changes sequentially. This concept is essential for understanding scenarios like compound interest or successive discounts.
Example: Apply two successive discounts of 10% and 20% on a product priced at \$100.
$$
\text{First Discount: } 100 - (100 \times 0.10) = 90 \\
\text{Second Discount: } 90 - (90 \times 0.20) = 72 \\
\text{Final Price: } \$72
$$
The overall discount is not 30% but 28% due to the compounding effect.
12. Solving Percentage Problems Using Equations
Many percentage problems can be solved by setting up equations based on the relationships between percentages, fractions, and decimals.
Example: If 25% of a number is 50, find the number.
Let the number be $x$.
$$
0.25x = 50 \\
x = \frac{50}{0.25} = 200
$$
Therefore, the number is 200.
13. Percentage in Geometry
Percentages are used in geometry to calculate aspects like area and volume increases or decreases. For example, determining the percentage increase in the area of a shape when its dimensions are scaled by a certain factor.
Example: A square has its side length increased by 20%. Find the percentage increase in its area.
$$
\text{Original Area} = s^2 \\
\text{New Side Length} = s \times 1.20 \\
\text{New Area} = (s \times 1.20)^2 = s^2 \times 1.44 \\
\text{Percentage Increase} = (1.44 - 1) \times 100\% = 44\%
$$
14. Common Mistakes to Avoid
- Misplacing Decimal Points: Ensure correct placement when converting between percentages and decimals.
- Incorrect Fraction Simplification: Always simplify fractions to their lowest terms.
- Overlooking Compound Effects: When dealing with compound percentages, apply each percentage step by step.
- Assuming Linear Relationships: Recognize that percentage changes are relative and not always additive.
Being aware of these common pitfalls helps in achieving accurate calculations and understanding.
Comparison Table
Aspect |
Percentage |
Fraction |
Decimal |
Definition |
Part per hundred |
Numerator over denominator |
Base-10 representation |
Symbol |
% |
½, ¾, etc. |
0.5, 0.75, etc. |
Conversion to Each Other |
Divide by 100 to get decimal; multiply by denominator to get fraction |
Multiply by 100 to get percentage; divide numerator by denominator for decimal |
Multiply by 100 to get percentage; recognize denominator as power of 10 for fraction |
Usage |
Comparing relative sizes, discounts, interest rates |
Expressing parts of a whole, ratios |
Calculations involving measurement, precision |
Advantages |
Easy to understand, widely used in real life |
Precise representation of ratios |
Facilitates arithmetic operations |
Limitations |
Can be misleading without context |
May require simplification for better understanding |
Less intuitive for some comparisons |
Summary and Key Takeaways
- Percentages, fractions, and decimals are interchangeable representations of parts of a whole.
- Mastering conversions between these forms is essential for solving various mathematical problems.
- Understanding compound percentages enhances problem-solving in real-life scenarios.
- Avoid common mistakes by ensuring accurate calculations and proper simplifications.