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Open Number Line

Rounding Numbers

Grade 3

Created by:

Math & Movement

Estimated Activity Time:

Standard:

Concept(s):

Activity Instructions

(1) Place the multiples of ten on every other hash mark, starting with 0 and ending with 50. Discuss finding the “half-way” point between 0 and 10 (depending on the students’ level, you may even want to have ten children stand up and divide them into two equal groups to show that half of 10 is 5). Place a colored index card with the number “5” halfway between 0 and 10. Have the same discussion and make connections for all the halfway points between each multiple of ten.

(2) The open number line should now have all the multiples of five from 0-20 on the number line.

(3) Explain that “rounding” numbers to the nearest ten is finding the closest multiple of ten to a number. Rounding can be used to find the estimated answer to an equation.

(4) Create number cards on notecards with numbers 0 to 50. Give students a number card and have them place it on the number line where they think it should go based on the multiples of five and ten already on the number line. Ask the student, “What multiple of ten, number ending in a zero, is closest to your number? Is your number before or after the halfway point between each multiple of ten? If on or after the halfway point, round to the higher ten.”

(5) Next, have students solve addition problems by estimating the sum. First, they will need to round each addend to the closest multiple of ten, and then add the rounded numbers together. Students can also practice with subtraction. (Worksheets linked below.)

(6) Change the number line so it starts at 100 and ends at 200. Repeat Step #1. Then repeat steps #4 and #5.

(7) Ask students to round a number to the closest multiple of one hundred by labeling every other hash mark with a multiple of one hundred. Discuss that half way between each 100 would be 50. Then, give students a number card such as 235. This number comes before 250 (the halfway point between 200 and 300), thus the number 235 would round to 200.

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