Automaticity subtraction to 100

Learn to quickly and correctly subtract numbers to 100 in your head

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Students learn to quickly and correctly subtract numbers to 100 in their heads.

CCSS.Math.Content.2.NBT.B.5

Students will be able to quickly and correctly subtract numbers to 100 in their head.

Practice two subtraction problems with the students by solving them using the visual support provided on the interactive whiteboard.

Explain to students what automaticity is, and what automaticity in subtraction problems means. Tell students that they are going to practice solving subtraction problems in their head within a given time. The goal is to solve the subtraction problem within 5, 4, and even 3 seconds. Start by discussing the three problems, the first in 5 seconds, then 4, and finally in 3 seconds. Then ask students to solve the problems within those times and discuss the answers. Tell students that have trouble solving within time that this is something that improves with practice. Then challenge the class to solve the set of three problems within 15 seconds and then four problems within 20 seconds.

Check that students are able to quickly and correctly solve subtraction problems to 10 by asking the following questions:

- Why is it useful to be able to quickly solve subtraction problems?

- How can you make sure that you get better and faster at solving subtraction problems?

Students practice with sets of subtraction problems and are challenged to complete them in progressively less time.

Students are given subtraction problems in the spoken form. Ask students if they can solve all five problems in 25 seconds. Discuss the answers students got with the actual answers and ask students to explain their strategy. Then play Subtraction Speedway. Split the class into 2 groups. One student goes to stand in the middle and comes up with a subtraction problem. The team that comes up with the correct answer first wins a point. After earning three points the student in the middle gets exchanged for another. You may also modify this to have multiple smaller groups, depending on your class size.

Students who have difficulty solving the subtraction problems within 5 seconds can first be given the task of practicing subtraction problems to 100 without any time limits. Then you can start by having them solve a problem in 10 seconds, 8 seconds, 5 seconds, etc.

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