September 18, 2017
|
by Byron Delcomb

Lesson plan

Single Strategy for Adding and Subtracting Mixed Numbers

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Do you need extra help for EL students? Try the Explain Fraction Conversions pre-lesson.
EL Adjustments

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Do you need extra help for EL students? Try the Explain Fraction Conversions pre-lesson.

Students will be able to add and subtract mixed numbers by converting them into improper fractions and back again.

The adjustment to the whole group lesson is a modification to differentiate for children who are English learners.
EL adjustments
(5 minutes)
  • Show your class two examples:
    • a whole piece of fruit and ¼ slice to represent 1 ¼,
    • another model of 1 and ¼ of fruit, where the whole has been partitioned into ¼ pieces.
  • Ask your class which would be easier to subtract ¾ from and why. Have your students turn to a neighbor and share their thoughts.
  • Allow students to share out to the whole class and tease out the notion that it's easier to subtract from whole items already in a group of like pieces.
  • Share that today's lesson uses a strategy for adding and subtracting mixed numbers by first making them improper fractions, or as a group of like or same-sized pieces.
  • Point out to your students that mixed numbers are different from improper fractions in that they include a whole number and a proper fraction (where the numerator is less than the denominator).
(10 minutes)
  • Show your class the following expression: 1 ⅖ + 3 ⅘.
  • Explain that there is a three-step sequence that will always work when adding or subtracting mixed numbers:
    • Step one: convert the mixed number to an improper fraction.
    • Step two: perform the operation.
    • Step three: convert the improper fraction back to a mixed number.
  • Review converting a mixed number to an improper fraction, and vice versa, as needed. (This is not the focus of this lesson, but these skills are required to proceed.)
  • Take your class through the three-step sequence for adding and subtracting mixed numbers for the following: 1 ⅖ + 3 ⅘.
    • Step one: convert the mixed numbers to improper fractions: 7/5 + 19/5.
    • Step two: perform the addition. The sum is 26/5.
    • Step three: convert the improper fraction: 26/5 becomes 5 ⅕.
(5 minutes)
  • Repeat the three-step sequence for 3 ⅘ – 1 ⅖, calling on various students for each step. Answer clarifying questions students may have.
  • Tell your students to take out their math journals and list the three-step sequence for adding and subtracting mixed numbers.
  • Post the following expression and perform the three-step sequence with the expression 1 6/8 + 8 2/8. Ensure students label the three steps in their solutions.
  • Answer any clarifying questions and post the following exercise problems:
    • 3 ⅝ + 1 ⅜
    • 4 ¼ – 1 ¾
    • 6 ⅚ + 3 ⅙
    • 5 ⅕ – 3 ⅘
    • 7 ⅓ – 2 ⅔
(15 minutes)
  • Instruct your class to copy and complete the exercise problems in their math journals.

Support:

  • For review on converting mixed numbers to improper fractions or converting improper fractions to mixed numbers, see the Related Media section for videos for each procedure.
  • Print the the three-step sequence for adding and subtracting mixed numbers in advance. Print five to a page and tear off in strips for students to place in their math journals.

Enrichment:

  • Students can add an extra addend as an addition challenge. For a subtraction challenge, have students design a subtraction expression that uses two subtrahends and works!
  • Use an interactive whiteboard to demonstrate the processes involved when converting mixed numbers to improper fractions and back again.
  • Assign exercise problems in your Google Classroom application for student reference and as a repository for their solutions.
(5 minutes)
  • Show students a mixed number subtraction expression with two potential answers.
  • Ask students to select the correct answer, including details for each of the three steps that yield the answer.
  • Repeat for a mixed number addition expression.
(15 minutes)
  • Review exercises by having students share out their answers with details of each stage in the three-step solution sequence.
  • Allow for students to "phone a friend" for assistance if they get stuck to take over their explanation.
  • Discuss the following question, "How efficient is this strategy and how does it compare to others?"
  • Post key learnings and comments on a poster for future reference.

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