standard_description: Add and subtract fractions with unlike denominators (including mixed numbers) by replacing given fractions with equivalent fractions in such a way as to produce an equivalent sum or difference of fractions with like denominators. For example, 2/3 + 5/4 = 8/12 + 15/12 = 23/12. (In general, a/b + c/d = (ad + bc)/bd.)
problem_activity: Narrative
The purpose of this activity is for students to equally divide sandwiches in situations where the number of portions does not evenly divide the number of sandwiches. Students select the number of people sharing and the number of sandwiches from a small set of deliberately chosen numbers. Depending on their choice, each share may be less than a full sandwich or more than a full sandwich. This encourages students to make sense of the problem and persevere in solving it (MP1). The focus of the synthesis is on different ways students choose to make equal shares and how they know that the shares are equal. Many of the strategies for dividing the sandwiches equally are examined in detail in the next activity. In this activity, students explain how they know the shares are equal. Student representations may include coloring to show the equal shares. Monitor and select students who use the following strategies to share in the synthesis:
choose numbers so each person gets one or two whole sandwiches and a fraction of a sandwich
choose numbers so each person gets less than a full sandwich
MLR1 Stronger and Clearer Each Time.
Synthesis: Before the whole-class discussion, give students time to meet with 2–3 partners to share and get feedback on their response to how much sandwich each person gets. Invite listeners to ask questions, to press for details and to suggest mathematical language. Give students 2–3 minutes to revise their written explanation based on the feedback they receive.
Advances: Writing, Speaking, Listening
Launch
Groups of 2
Display the prompt without the questions.
__________ sandwiches are shared equally by __________ people.
“What do you notice? What do you wonder?” (There are no numbers. How many sandwiches are there? How many people are there? What kind of sandwiches are they?)
“What are some numbers that would make sense? Why do those numbers make sense?” (One sandwich per person because that is a familiar serving size. Many people and many sandwiches because there is a party or event. More people than sandwiches because the people are sharing big sandwiches.)
Activity
2 minutes: independent think time
5–8 minutes: partner work time
As students work, consider asking:
“Where are the sandwiches in your drawing or diagram?”
“How does your diagram or drawing show how much sandwich each person will get?”
Student Facing
__________ sandwiches are shared equally by __________ people.
Choose numbers to fill in the blanks. You can only use each number once: 2, 3, 5.
Represent the situation with a diagram or drawing.
Explain or show how you know that each person will get the same amount of sandwich.
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Advancing Student Thinking
Students may recognize that the natural shape of their chosen sandwich impacts the size of certain pieces and, therefore, some people are getting a little bit more sandwich if they get a certain piece. When they recognize the constraints of the shape of a sandwich in real life, students are making sense of problems. Ask these students, “If you were going to share these sandwiches with your friends, which pieces would you want? Why would you want those pieces? Why do you think someone might choose the end pieces? Is it fair to say that each of you got about the same amount of sandwich?”
Activity Synthesis
Ask the selected students to display their responses side by side for all to see.
“Does each person get more than a full sandwich or less? How do you know?” (Less because there are more people than sandwiches or more because there are more sandwiches than people.)
“This situation is about sharing. We can represent equal sharing situations with division expressions. What division expressions can we write to represent each of the situations we represented?” (Answers vary depending on number choices:
\(2\div3\)
,
\(3\div2\)
,
\(2\div5\)
,
\(5\div2\)
,
\(3\div5\)
,
\(5\div3\)
)


aligns: no