Subtraction to 100 by creating an addition problem
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General
Students learn to create addition problems to help solve their subtraction problems with numbers to 100.
Common core standard(s)
2.NBT.B.5
2.OA.A.1
Relevance
Students learn that it is useful to create an addition problem to solve a given subtraction problem.
Introduction
Practice a subtraction problem with the students in which they must drag blocks from the total to a box and say what the subtraction problem is. Next ask students how many blocks you'd need to add to reach the total number- and tell them the addition problem this creates.
Development
Explain to students that it is useful to solve some subtraction problems by turning them into addition problems. Once the students learn how to create an addition problem from their subtraction problem, they can choose whether they want to subtract or add. Use the number line to explain how to create an addition problem from a subtraction problem and how to solve. Discuss how you add. The number added to the second number in the subtraction problem is the total, or first number of the subtraction problem. To check if students understand how to do this, ask them to solve the two given problems individually or in pairs.
Check that students are able to create an addition problem from a subtraction problem with numbers to 100 by asking the following questions:
- Why is it useful to turn a subtraction problem into an addition problem?
- How do you turn a subtraction problem into an addition problem?
!---Guided practice
Students are first given a subtraction problem that they need to turn into an addition problem and solve with visual support. They are then given problems that are without visual support.
Closing
Ask students a few questions to check their understanding of the learning goal. Then practice with subtraction problems, one with visual support and two in the abstract.
Teaching tips
For students who have difficulty with this learning goal, have them start with subtraction problems with numbers to 20. Walk through the steps of converting a subtraction problem to an addition problem and have students use a number line or manipulatives like blocks.
Instruction materials
Optional: blocks or other manipulatives
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