Grade 3Fractions3.NF.A.1

Building Fractions from Unit Fractions

3/4 is three pieces of size 1/4.

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A unit fraction is one piece. Now we put pieces together. $\frac{3}{4}$ means 3 pieces, each of size $\frac{1}{4}$.

Tap parts to shade them. Shade 3 of the 4 equal parts to show three fourths.

Why it works

The bottom number (the denominator) tells the size of each piece: fourths. The top number (the numerator) counts how many of those pieces you have. So $\frac{3}{4}$ is one amount, "three fourths," just as "3 apples" is one amount: the 3 counts and the fourths say what is being counted.
The numerator counts; the denominator names the piece.
FractionSayPieces
$\frac{3}{4}$three fourths3 pieces of $\frac{1}{4}$
$\frac{5}{6}$five sixths5 pieces of $\frac{1}{6}$
$\frac{2}{8}$two eighths2 pieces of $\frac{1}{8}$

Key idea

$\frac{a}{b}$ is $a$ pieces, each of size $\frac{1}{b}$.

Worked example: Shaded parts

A circle is cut into 8 equal parts and 5 are shaded. What fraction is shaded?
  1. Each part is $\frac{1}{8}$.
  2. 5 parts are shaded: 5 pieces of $\frac{1}{8}$.

Answer: $\frac{5}{8}$

Worked example: Count the pieces

How many pieces of size $\frac{1}{6}$ make $\frac{4}{6}$?
  1. The numerator, 4, counts the pieces.

Answer: $4$

Worked example: What is left?

3 of 5 equal parts are shaded. What fraction is not shaded?
  1. $5 - 3 = 2$ parts are not shaded.
  2. Each part is $\frac{1}{5}$, so 2 parts make $\frac{2}{5}$.

Answer: $\frac{2}{5}$

Common mistake

Reading the two numbers separately. $\frac{3}{4}$ is not "3 and 4" and it is not bigger than 3. It is one number, less than 1: three of the four equal parts. Say it aloud as "three fourths" to keep it together.

Practice

Pass with 3 right answers, the last 2 in a row. Each right answer earns 1 XP.

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Builds on

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Sources and further reading

  • Common Core State Standards for Mathematics, Grade 3, 3.NF.A.1 (understand a/b as the quantity formed by a parts of size 1/b)
  • OpenStax, Prealgebra 2e, section 4.1, Visualize Fractions
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