Finding This Percent of That

5 min

Narrative

This Math Talk focuses on the relationship between division by 100 and multiplication by 1100\frac{1}{100}. It encourages students to rely on what they know about place value, fractions, decimals, and the relationship between multiplication and division to mentally solve problems. The reasoning elicited here will be helpful when students work with percentages later in the lesson.

Launch

Tell students to close their books or devices (or to keep them closed). Reveal one problem at a time. For each problem:

  • Give students quiet think time, and ask them to give a signal when they have an answer and a strategy.
  • Invite students to share their strategies and record and display their responses for all to see.
  • Use the questions in the activity synthesis to involve more students in the conversation before moving to the next problem.

Keep all previous problems and work displayed throughout the talk.

Action and Expression: Internalize Executive Functions. To support working memory, provide students with sticky notes or mini whiteboards.
Supports accessibility for: Memory, Organization

Student Task

Find the value of each expression mentally.

  • 70÷10070 \div 100
  • 35110035 \boldcdot \frac{1}{100}
  • (0.35)100(0.35) \boldcdot 100
  • 105100\dfrac{105}{100}

Sample Response

  • 0.7 (or equivalent). Sample reasoning: 
    • 700÷100700 \div 100 is 7, so 70÷10070 \div 100 is one tenth of 7, which is 0.7.
    • 70÷10070 \div 100 is 70100\frac{70}{100}, or 710\frac{7}{10}, which is 0.7.
    • 70÷10070 \div 100 is equivalent to 70110070 \boldcdot \frac{1}{100}, which is 70100\frac{70}{100} or 0.70, or 0.7.
  • 0.35. Sample reasoning:
    • 35110035 \boldcdot \frac{1}{100} is 35 groups of one-hundredth, which is 35-hundredths.
    • 35110035 \boldcdot \frac{1}{100} is the same as 35÷10035 \div 100, which is 0.35.
    • 70÷10070 \div 100 or 70110070 \boldcdot \frac{1}{100} is 0.7.  Because 35 is half of 70, and 35110035 \boldcdot \frac{1}{100} is half of 0.7, which is 0.35.
  • 35. Sample reasoning:
    • 35÷10035 \div 100 is 0.35, so (0.35)100(0.35) \boldcdot 100 is 35.
    • (0.35)10(0.35) \boldcdot 10 is 3.5, so (0.35)100(0.35) \boldcdot 100 is 10 times 3.5 or 35. 
    • 0.35 is 35100\frac{35}{100}, and 35100100=3,500100\frac{35}{100} \boldcdot 100 = \frac{3,500}{100}, which is 35.
  • 1.05 (or equivalent). Sample reasoning:
    • 100100\frac{100}{100} is 1 and 5100\frac {5}{100} is 0.05, so 105100\frac{105}{100} is 1.05.
    • 105100\frac{105}{100} is 105÷100105 \div 100, which is 1.05.
    • (1.05)100(1.05) \boldcdot 100 is 105.

Synthesis

To involve more students in the conversation, consider asking:

  • “Who can restate \underline{\hspace{.5in}}’s reasoning in a different way?”
  • “Did anyone use the same strategy but would explain it differently?”
  • “Did anyone solve the problem in a different way?”
  • “Does anyone want to add on to \underline{\hspace{.5in}}’s strategy?”
  • “Do you agree or disagree? Why?”
  • “What connections to previous problems do you see?”

Highlight two points: that dividing a number by 100 gives the same result as multiplying the number by 1100\frac{1}{100}, and that a fraction can be interpreted as division (105100\frac{105}{100} can be understood as 105÷100105 \div 100).

MLR8 Discussion Supports. Display sentence frames to support students when they explain their strategy. For example, “First, I \underline{\hspace{.5in}} because . . . .” or “I noticed \underline{\hspace{.5in}} so I . . . .” Some students may benefit from the opportunity to rehearse what they will say with a partner before they share with the whole class.
Advances: Speaking, Representing
Standards
Building On
  • 5.NBT.6·Find whole-number quotients of whole numbers with up to four-digit dividends and two-digit divisors, using strategies based on place value, the properties of operations, and/or the relationship between multiplication and division. Illustrate and explain the calculation by using equations, rectangular arrays, and/or area models.
  • 5.NBT.B.6·Find whole-number quotients of whole numbers with up to four-digit dividends and two-digit divisors, using strategies based on place value, the properties of operations, and/or the relationship between multiplication and division. Illustrate and explain the calculation by using equations, rectangular arrays, and/or area models.

15 min

15 min