Lesson 15

The purpose of this Choral Count is for students to practice counting by 10 and 100 and notice patterns in the count. These understandings help students develop fluency and will help students see that multiples of 100 are also multiples of 10, and prepare them to round large numbers to the nearest ten and hundred.

• “Count by 10, starting at 0.”
• Record as students count.
• Stop counting and recording at 200.
• “Count by 100, starting at 0.”
• Record as students count.
• Stop counting and recording at 900.

• “What patterns do you see?”
• 1–2 minutes: quiet think time
• Record responses.

• “Why do 100 and 200 show up in both of these counts?” (You say 100 and 200 when you are counting by tens or hundreds. You can think of 100 as 10 tens or 200 as 20 tens. 100 and 200 are multiples of 10 and multiples of 100.)

The purpose of this activity is for students to round given numbers to the nearest ten and hundred and see that the result can be the same for some numbers. Students think about what it means to round a number that is exactly halfway between two tens or two hundreds and are introduced in the synthesis to the convention that these numbers are rounded up (MP3).

• Groups of 2
• “Let’s round some numbers to the nearest ten and the nearest hundred. Remember ‘round to the nearest ten (or hundred)’ is another way of saying find the nearest multiple of 10 (or 100).”

• “Work with your partner to complete the table.”
• 3–5 minutes: partner work time
• Monitor for students who notice 97 and 601 round to the same number whether rounding to the nearest ten or the nearest hundred.
• Select previously identified students to share their responses.
• “Why did 97 and 601 round to the same number when we rounded to the nearest ten and the nearest hundred?” (The closest multiple of 10 was also a multiple of 100. 100 and 600 are multiples of 100, but they are also multiples of 10.)
• “Complete the second problem with your partner. Explain your reasoning.”
• 3–5 minutes: partner work time
• Monitor for the following in student explanations to share in the synthesis:
  • 415 is halfway between 410 and 420.
  • 415 is the same distance from 410 and 420.
  • 415 has two closest multiples of 10: 410 and 420.
  • 750 is halfway between 700 and 800.
  • 750 is the same distance from 700 and 800.
  • 750 has two closest multiples of 100: 700 and 800.

• Select previously identified students to share their responses.
• “Kiran and Priya are correct. There isn’t one closest multiple of 10 to 415 because it’s right in the middle of two multiples of 10. There isn’t a closest multiple of 100 to 750 because it’s right in the middle of two multiples of 100.”
• “In cases like this, we round up. For example, since 415 is halfway between 410 and 420, we would round up to 420 to round 415 to the nearest ten.”
• “Since 750 is halfway between 700 and 800, we would round up to 800.”
• ”In both of these situations we could go the other way, but it is helpful if we all do the same thing in these situations.”

The purpose of this activity is for students to practice rounding to the nearest ten and hundred in context. Students work with numbers from a previous lesson to estimate the total number of students in a school. They learn that how you round (to the nearest ten or hundred) can give different estimates for the same situation.

• Groups of 2
• “Let’s revisit the number of students in different areas in a school. We’re going to use rounding to help Andre and Lin make an estimate of the number of people in the whole school. Andre plans to round the numbers to the nearest hundred. Lin plans to round them to the nearest ten.”
• “Make a prediction: Whose estimate is going to be greater? Be prepared to explain your reasoning.”
• 30 seconds: quiet think time
• Share responses.

• “Work with your partner to complete the table and the last problem.”
• 7–10 minutes: partner work time

• Invite students to share their observations about the completed table, including whether it matched their predictions.
• “When we are estimating a total, rounding to the nearest ten or the nearest hundred can give us different results. If you were rounding to estimate a total, would you round to the nearest ten or nearest hundred?” (The nearest ten because you get an estimate that’s closer to the total. The nearest hundred because multiples of 100 are easier to work with.)
• “Keep these ideas in mind as you use rounding to estimate in future lessons.”