Anthill Number Round-Off
The purpose of this lesson is to give students a visual aid to help them round off numbers to the
nearest ten or hundred.
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
Materials/resources
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blackboard or white marker board
chart paper
colored markers
Round Off: bmp | gif
Ant Hill diagram: bmp | gif
Pre-activities
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.
Activities
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.
Assessment
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
Comments
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
Number and Operation in Base Ten – Second Grade
Understand place value
2.NBT.2 Count within 1000; skip-count by 5s, 10s, and 1000s.
Number and Operation in Base Ten - Third Grade
Use place value understanding and properties of operations to perform multi-digit
arithmetic
3.NBT.1 Use place value understanding to round whole numbers to the nearest 10 or 100.
http://www.learnnc.org/lp/pages/3420