A8 — Estimating, Finding Landmark Lengths, and Rounding

Problem Strings for Fluency and Beyond · Estimating, Finding Landmark Lengths, and Rounding

Teacher notes

This string is like A7. As children work through these next several strings, they will be asked to determine the landmark neighbors and the distances to them (for example 8 is between 0 and 10 but is 8 units away from 0 and only 2 units away from 10, so 10 is the closest landmark neighbor. These strings will continue to support the development of number relationships while also building a foundation for the eventual understanding of the rules of rounding. For this string, show one number at a time and ask, "What landmark numbers are my neighbors and how far from each am I? As students share their responses, show each response on a separate number line.

Note how the first three pairs each have two numbers that are closest to the same landmark number: 8 and 12 are both 2 units away from 10; 16 and 24 are both 4 units away from 20; and 33 and 27 are both 3 units away from 30. The pattern stops with 65, 85, and 95. As students realize the last 3 numbers are equidistant to both landmarks, explain that adult mathematicians were perplexed, too, about what to do in these cases where the last digit on the right was a 5. So, they all agreed to always round up to the greater value in these cases.

The string