Lesson 9

• Display one statement.
• “Give me a signal when you know whether the statement is true and can explain how you know.”
• 1 minute: quiet think time

• Share and record answers and strategy.
• Repeat with each statement.

• “How can you explain your answer without finding the value of both sides?”
• Consider asking:
  • “Who can restate ___ ’s reasoning in a different way?”
  • “Does anyone want to add on to _____ ’s reasoning?”

In this activity, students reason about differences of fractions on a number line and write equations for number line diagrams that represent subtraction. They subtract a fraction from another fraction, as well as a whole number from a fraction, applying what they know about equivalence of whole numbers and fractions to facilitate their reasoning. When students decide whether or not they agree with Noah and explain their reasoning, they critique the reasoning of others (MP3).

In earlier grades, students used number lines to reason about subtraction of whole numbers. To find the value of \(42 - 15\), for example, they could start at 42 and jump 15 spaces to the left (or jump 2 spaces to 40, and then 10 spaces to 30, and 3 more to 27). They could also think in terms of addition—“What number added to 15 gives 42?”—and start at 15 and see how many spaces it takes to get to 42.

Students may reason about subtraction of fractions the same way here. For instance, to find \(\frac{8}{3} - \frac{2}{3}\), they may:

• Groups of 2
• Display the first three number line diagrams in the activity.
• “How are these diagrams the same as the diagrams we saw in an earlier lesson? How are they different?” (Same: They use jumps to show a change. Each space between tick marks represents a unit fraction. Different: There is only one jump. The arrows point to the left.)
• 1 minute: quiet think time
• Share responses.
• “How do we know that the point represents \(\frac{11}{6}\)?” (Each space represents 1 sixth. The point is 11 sixths from 0.)

• “Each of Noah's diagrams represents subtraction from \(\frac{11}{6}\). Think about what number is being subtracted and what the result of the subtraction might be.”
• “Work with your partner on the first two problems.”
• 5–7 minutes: partner work time
• Invite students to share their responses to the first problem.
• “Where do you see the numbers being subtracted?” (The number of spaces jumped)
• “Where do you see the result of the subtraction?” (The point where the arrow lands)
• For the second problem, poll the class on which equations they thought the diagram could represent (for example: only the first, only the second, only the third, the first two, all three, and so on). Invite students from each camp to share their reasoning.
• Make sure students recognize why the diagram can represent all three equations. (See Student Responses.)
• 2 minutes: independent work on the last problem

• Focus the discussion on the last expression \(\frac{8}{3} – 1\).
• “How did you subtract 1, a whole number, from \(\frac{8}{3}\), a fraction?” (Start at \(\frac{8}{3}\) and jump to the left 3 thirds, to land at \(\frac{5}{3}\). Start at 1 and find out how far to jump to the right to reach \(\frac{8}{3}\).)
• “How could you subtract 1 from \(\frac{8}{3}\) if you didn’t have a number line?” (I could:
  • Think of 1 as \(\frac{3}{3}\) and subtract \(\frac{3}{3}\) from \(\frac{8}{3}\), which gives \(\frac{5}{3}\).
  • Think about how many thirds to add to \(\frac{3}{3}\) to get \(\frac{8}{3}\).
  • Think of \(\frac{8}{3}\) as \(2\frac{2}{3}\) and subtract 1 from it, which gives \(1\frac{2}{3}\).)

In this activity, students use number lines to represent subtraction of a fraction by another fraction with the same denominator—including a mixed number—and by a whole number. Locating a fraction greater than 1 on the number line prompts students to decompose the fraction mentally into a whole number and a fractional part, rather than to rely on counting tick marks. Representing subtraction of a whole number on the number line encourages students to use their knowledge of whole-number equivalents of fractions and to look for and make use of structure (MP7). For example, when subtracting 1 from \(\frac{13}{8}\), it helps to think of 1 as \(\frac{8}{8}\), and when subtracting 1 from a mixed number such as \(1\frac{5}{8}\), it helps to notice that \(1\frac{5}{8}\) is \(1 + \frac{5}{8}\).

As before, students may reason about subtraction in terms of removing an amount or finding an unknown addend, resulting in different number line diagrams. While students may rely on number lines to find each difference, the reasoning they do here prompts them to notice patterns and to think flexibly, preparing them to reason numerically in upcoming lessons.

• Groups of 2

• “Take a few quiet minutes to work on the task. Then, share your responses with your partner.”
• 5–7 minutes: independent work time
• 2–3 minutes: partner discussion
• Monitor for the different strategies students use to locate \(\frac{13}{8}\) on the number line and to represent subtraction by 1 and \(1\frac{4}{8}\).

• Select students to share their responses and completed diagrams.
• Focus the discussion on expressions that involve subtraction by a whole number or by a mixed number.
• Display the third and fifth expressions side by side: \(\frac{13}{8} - 1\) and \(1\frac{5}{8} - 1\).
• “Did you use the same strategy to represent these expressions on the number line and to find the value? If so, what was your strategy? If not, what was different?”
• Display the fourth and last expressions side by side: \(1\frac{5}{8} - \frac{4}{8}\) and \(1\frac{5}{8} - 1\frac{4}{8}\). Ask students to compare how they found the value of these expressions.
• See lesson synthesis.

This optional activity gives students an additional opportunity to practice using “jumps” on number lines to subtract fractions, decomposing any whole numbers as needed along the way. (It is similar in structure to an optional activity in an earlier lesson on addition.)

Students are given four number lines with a point marked on each. They then draw a card with a fraction on it. All fractions on the cards, shown here, are greater than the values of the points. Students make one or more jumps to find the difference of the two points and then represent it with a subtraction equation. 

\(\frac{8}{5} \qquad \frac{9}{5} \qquad \frac{10}{5} \qquad \frac{11}{5} \qquad \frac{12}{5} \qquad \frac{13}{5} \qquad \frac{14}{5} \qquad \frac{15}{5}\)

• Create a set of cards from the blackline master for each group of 2.

• Groups of 2
• Give each group a set of fraction cards from the blackline master.
• If students did not complete the optional activity in the previous lesson (which had a similar structure):
  • Display the four number lines.
  • “What do you notice? What do you wonder?”
  • 30 seconds: quiet think time
  • 30 seconds: partner discussion

• “Label each point on the number line with a fraction it represents. This is your target.”
• 1 minute: independent work time
• “You will now make one or more jumps from another fraction to your target and write an equation to represent the difference between the two.”
• Explain how to use the cards and how to complete the task.
• Monitor for students who facilitate subtraction by:
  • rewriting a fraction on their card as a whole number or a mixed number
  • labeling their number lines with whole numbers beyond 1

• Select students to share their responses to the first couple of diagrams (or more if time permits).