K-12 Teaching and Learning From the UNC School of Education

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Learning outcomes

The learner will estimate by rounding off numbers to the nearest 10 or 100 by visually placing the number on an anthill. This visual method of seeing where the number “falls” on the anthill will enable the learner to understand the concept of rounding to the nearest multiple.

Teacher planning

Time required for lesson

1 hour


  • blackboard or white marker board
  • chart paper
  • colored markers
  • Round Off: bmp | gif
  • Ant Hill diagram: bmp | gif


The learners will be able to count by tens to 100, and by hundreds to 1,000. They should be familiar with number lines and be able to locate numbers on them.


  1. Tell the class they will learn to round off numbers to the nearest 10. Explain that rounding off numbers will help them in many ways. Give an example of how rounding numbers is used “in the real world”. I tell a story about a small boy who found himself in an embarrassing situation when he tried to buy snacks at a neighborhood store and discovered that he didn’t have enough money to pay for the items he wanted. I explain how he could have prevented his problem if he had known how to round off numbers and add them in his head.
  2. Review counting by tens to 100. Display the anthill that you have drawn on the chalkboard or on chart paper. (Refer to the attached anthill diagram). Explain that you will show them how to round off any two-digit number by first putting it on the anthill. Choose a two-digit number as an example and put it on the appropriate place on the ant hill. Write the multiples of ten that the number is between at the base of the ant hill, one on either side. Explain that if the number fell off the ant hill, it would fall down to the bottom of the hill on the side it was nearest. (Example: the number 33 would fall down on the side of 30 because it was closest.) The multiple of ten it falls near is the ten that number rounds to.
  3. Practice many more examples of rounding two-digit numbers in this manner. When students seem to understand the concept, ask if there is a rule that they can make up about rounding numbers by looking at the digit in the ones place. They should be able to tell you that when the digit in the ones place is four or smaller, the number rounds to the smaller ten. If the digit in the ones place is five or greater, it rounds to the larger ten.
  4. Distribute the worksheet for additional practice that students will complete independently (see attached). Leave the anthill displayed for students to reference. A few students may need additional help until they have “caught on”.
  5. At the conclusion of the lesson, students draw an anthill in their math journals and explain how it is used to round off numbers.

The same procedure can be used to round off three-digit numbers to the nearest hundred. Use the digit in the tens place to determine which side of the anthill the number falls to.


The teacher will assess students’ learning by questioning and observation during the lesson. The independent practice provided by the attached worksheet can also indicate skill mastery.

Supplemental information


It is sometimes helpful to allow students to work together in small groups to complete this lesson. They have an amazing ability to teach each other.

  • Common Core State Standards
    • Mathematics (2010)
      • Grade 2

        • Number & Operations in Base Ten
          • 2.NO.2 Count within 1000; skip-count by 5s, 10s, and 100s.
        • Grade 3

          • 3.NO.1Use place value understanding to round whole numbers to the nearest 10 or 100.

North Carolina curriculum alignment

Mathematics (2004)

Grade 3

  • Goal 1: Number and Operations - The learner will model, identify, and compute with whole numbers through 9,999.
    • Objective 1.01: Develop number sense for whole numbers through 9,999.
      • Connect model, number word, and number using a variety of representations.
      • Build understanding of place value (ones through thousands).
      • Compare and order.