B1.7 Convert between fractions, decimal numbers, and percents, in various contexts.
Skill: Converting Between Fractions, Decimals and Percents
In order to develop number sense students should form mental representations of quantities and draw on their knowledge of fractions (for example, 0.75 is read as "75 hundredths", not "zero point 75" ). Students should be encouraged to use multiple models to help create various mental representations that show the relationships between fractions and decimals.
When representing decimal numbers using models, there is an adjustment to be made since these same models were previously used differently. For example, where the rod represented ten ones, it might be representing one whole when exploring decimals. Students must understand that the unit or value assigned to each block has changed. In the examples above, the rod (whole) represents 10 tenths, and each cube is one tenth. The large square (whole) represents 100 hundredths, so each small square is one hundredth.
Students must also form a mental representation of decimal numbers greater than one. When reading such a decimal number, they must mentally represent the quantity it represents by interpreting the two parts that make it up: the whole number part and the decimal part. For example, they must recognize that the number 8.24 represents 8 wholes and a part of another identical whole. They can then visualize a quantity between 8 and 9.
Examples
Over time, students are able to mentally picture the quantity in context. For example, in a situation involving an item that costs $197.98, a student who recognizes that $197.98 is almost a dollar more than $197.00 and a little bit less that $198.00, can then visualize or conceive that, in this context, the amount of $197.98 can be represented approximately by ten $20 bills.
Students need to be able to create a mental representation of a percent, as they do with decimal numbers. The very nature of a percent makes it easier for them to visualize a quantity, as it is always a ratio to 100. It should also be understood that a percent is another way of representing a quantity.
Example
Benchmarks
The mental representations used by students are reinforced by the use of benchmarks. In general, a benchmark is a reference point. The benchmarks used in the study of decimals and percents are similar to those used in the study of fractions. By making connections between decimals, percents, and fraction benchmarks, students deepen their number sense.
The following table provides some benchmarks that students should know.
Benchmarks for Fractions, Percents and Decimals
≈: mathematical symbol meaning "is approximately equal to
| Fraction | Percent | Decimal Number | Example of a Mental Representation |
|---|---|---|---|
| \(\frac{1}{{10}}\) | 10% | 0.1 |  |
| \(\frac{1}{4}\) | 25% | 0.25 |  |
| \(\frac{1}{3}\) | \( \approx \;0.33% \) | \( \approx \;0.33\; \) |  |
| \(\frac{1}{2}\) | 50% | 0.5 |  |
| \(\frac{2}{3}\) | \( \approx \;67% \) | \( \approx \;0.67 \) |  |
| \(\frac{3}{4}\) | 75% | 0.75 |  |
| 1 | 100% | 1.00 |  |
These benchmarks, as well as the connections between fractions, percents and decimals, help to deepen number sense and are very useful in problem-solving situations. The ability to switch from one notation to another is advantageous because it allows students to use the one that best meets the needs of the moment. For example, a customer who wants to calculate a 50% discount on the price of an item can easily do so if he recognizes that 50% is half of \(\frac{1}{2}\).
Source: translated from Guide d'enseignement efficace des mathématiques de la 4e à la 6e année, Numération et sens du nombre, Fascicule 3, Nombres décimaux et pourcentages, p. 36-40.
Equivalence and Equality Relationships
It is important to recognize equalities between various representations of numbers or numerical expressions. In the context of decimal numbers, students should recognize the equality between a decimal number, the corresponding decimal fraction, and the percent.
Students need to understand that since decimal notation is just another way of representing a decimal fraction, it is then possible to establish an equality relationship between the two notations (for example, \(0.3\; = \;\frac{3}{{10}}\)). By recognizing this equality, students are better able to associate a place value with each of the digits (for example, tenths, hundredths) that make up a decimal number.
Example
Students who do not understand this association are sometimes inclined to represent a fraction such as \(\frac{2}{5}\) by 0.2 or 0.25. The following description describes observable behaviours of students who have developed a conceptual understanding of numbers and the relationship between their different representations.
Source: translated from Guide d'enseignement efficace des mathématiques de la 4e à la 6e année, Numération et sens du nombre, Fascicule 3, Nombres décimaux et pourcentages, p. 46.
Conceptual Understanding of Numbers
Number: 0.3
Observable behaviours:
- The student reads it as three tenths.
- The student can write the number in fractional notation as \(\frac{3}{{10}}\).
- The student can represent the number using concrete materials or visuals.
Example
Number: \(\frac{16}{100}\)
Observable behaviours:
- The student reads it as sixteen hundredths.
- The student can write the number in decimal notation as 0.16.
- The student can represent the number using concrete materials or visuals.
Example
Number: \(\frac{2}{7}\)
Observable behaviours:
- The student reads it as two sevenths.
- The student knows that the fraction \(\frac{2}{7}\) is not represented by 0.2 since they know that 0.2 = \(\frac{2}{10} \).
To establish the relationship between a fraction whose denominator is not a power of 10 (for example, \(\frac{1}{4}\)) and the corresponding decimal number, it is necessary to use the concept of equivalent fractions. For example, students can use strips of equal length as shown below to see that \(\frac{1}{4}\) is between\(\frac{2}{{10}}\) and \(\frac{3}{{10}}\).
They can then subdivide the tenths into 10 equal parts, thus creating 100 equal parts, or hundredths of the whole, and recognize that \(\frac{{25}}{{100}}\) is a fraction equivalent to \( \frac{1}{4}\).
Since \(\frac{{25}}{{100}}\; = \;0.25\), they can conclude that the fraction \(\frac{1}{4}\) can also be represented in decimal notation by 0.25 (\(\frac{1}{4}\; = \;\frac{{25}}{{100}}\; = \;0.25\)). This kind of example allows students to recognize that all fractions that can be expressed by an equivalent decimal fraction can be represented by a decimal number. This is particularly the case for fractions expressed in halves, fourths, fifths and twentieths, as shown in the following table.
| Fraction | Equivalent Decimal Fraction | Decimal Number |
|---|---|---|
| \(\frac{1}{2}\) | \(\frac{5}{10}\) | \(0.5\) |
| \(\frac{3}{4}\) | \(\frac{{75}}{{100}}\) | \(0.75\) |
| \(\frac{2}{5}\) | \(\frac{4}{{10}}\) | \(0.4\) |
| \(\frac{7}{{20}}\) | \(\frac{35}{{100}}\) | \(0.35\) |
Note: Some fractions (for example, \(\frac{2}{3}\), \(\frac{3}{7}\), \(\frac{5}{{11}}\)) cannot be represented by an equivalent decimal fraction. They can, however, be expressed by decimal numbers with a repeating decimal part (for example, \(\frac{2}{3}\; = \;0.\dot{6}\); \(\frac{3 }{7}\; = \;0.\dot{4}\dot{2}\dot{8}\dot{5}\dot{7}\dot{1}\); \(\frac{5}{{11}}\; = \;0.\dot{4}\dot{5}\)), obtained by dividing the numerator by the denominator.
Source: translated from Guide d'enseignement efficace des mathématiques de la 4e à la 6e année, Numération et sens du nombre, Fascicule 3, Nombres décimaux et pourcentages, p. 46-48.
A decimal number represents a fraction whose denominator is a power of 10 (for example, \(\ 0.3 = \frac{3}{10} \) ; \(\ 0.47 = \frac{{47}}{{100}}\)). Because the concept of percent is so closely related to the concept of fraction, it is only one more step to connect percent, decimal number, and fraction. By the end of the intermediate grades, students who have developed good number sense can move from one notation to another without difficulty.
Example
To help students develop this skill, they should be regularly invited to express their answers using another notation. For example, teachers can encourage the student who answered that \(\frac{3}{4}\) of the young people in the class have black hair to also express this answer in decimal notation (0.75) and as a percentage (75%).
Source: translated from Guide d'enseignement efficace des mathématiques de la 4e à la 6e année, Numération et sens du nombre, Fascicule 3, Nombres décimaux et pourcentages,, p. 50-51.
Knowledge: Percent
Percent is a special kind of fraction. It is often used in everyday life. A numerical expression such as 30% (which reads "30 percent") is actually another representation of the number 30 hundredths, or 0.30. To facilitate understanding of the concept of percent, students should first make the connection between percent and a fraction with a denominator of 100, using concrete or visual (semi-concrete) materials.
Example
Students should also realize that a percent represents a ratio to 100 (for example, 30% is the ratio of 30:100). It is important to note that a result expressed as a percent does not mean that the quantity in question is necessarily composed of 100 parts, as explained in the following table.
Relationship Between Percent and the Quantity 100
| Representation | Percent | Instructional notes |
|---|---|---|
 
|
75% of the circles are green. | Even if 75% of the circles are green, it does not mean that there are 100 circles in the whole. However, if there were 100 circles, there would be 75 green circles. Also, the fraction of circles that are green is equivalent to \(\frac{{75}}{{100}}\) (for example, \(\frac{3}{4}\; = \;\frac{75}{{100}}\) and if this amount were doubled the corresponding fraction would be: \(\frac{{150}}{{200}}\; = \;\frac{75}{100}\). |
|
50% of the land is covered with grass. | Even if 50% of the field is covered with grass, it does not mean that the field has an area of 100 m2. The entire field is 4000 m2 . If half of it is covered with grass, there would be 2000 m2 of grass. Thus, \(\frac{{2000}}{{4000}}\; = \;\frac{1}{2}\; = \;\frac{{50}}{{100}}\; = \;50\% \). |
Source: translated from Guide d'enseignement efficace des mathématiques de la 4e à la 6e année, Numération et sens du nombre, Fascicule 3, Nombres décimaux et pourcentages, p. 34-35.