Round Numbers

10 min

Narrative

This Number Talk encourages students to think about the distance of a number to a multiple of 100, 1,000, and 10,000 by relying on the structure of numbers in base-ten to mentally find differences (MP7). The understanding elicited here will be helpful later in the lesson when students round multi-digit whole numbers. It may be helpful to record students' reasoning on number lines.

Launch

  • Display one equation.
  • “Give me a signal when you have an answer and can explain how you got it.”
  • 1 minute: quiet think time
Teacher Instructions
  • Record answers and strategy.
  • Keep equations and work displayed.
  • Repeat with each equation.

Student Task

Find the value that makes each equation true mentally.

  • 421+‾=500421 + \underline{\hspace{1in}} = 500
  • 421+‾=1,000421 + \underline{\hspace{1in}} = 1,000
  • 6,421+‾=7,0006,421 + \underline{\hspace{1in}} = 7,000
  • 6,421+‾=10,0006,421 + \underline{\hspace{1in}} = 10,000

Sample Response

  • 79: 420 is 80 away from 500, so 421 must be 79 away from 500.
  • 579: 1,000 is 500 away from 500, and 500 is 79 away from 421.
  • 579: 6,421 is 79 away from 6,500, and 6,500 is 500 away from 7,000.
  • 3,579: it's like the last one but you need to add another 3,000 to get from 7,000 to 10,000.

Synthesis

  • “What do all four equations have in common?” (They are about finding how far away a number is to a multiple of 100, 1,000, or 10,000. The number on the left side of the equation is 421 or ends with 421.)
  • “Is 500 the nearest multiple of 100 for 421? How do you know?” (No, 421 is closer to 400. It is 21 away from 400 and 79 from 500.)
  • “Is 7,000 the nearest multiple of 1,000 for 6,421?” (No, 6,421 is closer to 6,000.)
  • “Is 10,000 the nearest multiple of 10,000 for 6,421?” (Yes, 10,000 is less than 5,000 away from 6,421.)
Standards
Addressing
  • 4.NBT.3·Use place value understanding to round multi-digit whole numbers to any place.
  • 4.NBT.A.3·Use place value understanding to round multi-digit whole numbers to any place.

20 min

15 min

20 min