# Lesson: When to Regroup

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### Lesson Objective

Students will determine how and when to regroup in subtraction problem.

### Lesson Plan

Do Now

• Tell students to solve the following problem in their math journals : 859 - 523.

Opening:

• State today's objective
• Ask for volunteer(s) to explain how they solved the Do Now problem.

Teach Input

• Using unifix cubes as you would base ten blocks, model the subtraction problem.
• Write a problem on the board that requires regrouping. (i.e. 764-189).
• Ask students to talk to their face partner about ways to solve that problem.
• Show students that they could take ten ones from the tens place and add them to the ones place. Model how to solve this problem on the board.
• Show that the subtractionÂ remains the same.

Guided Practice

• Ask two students to come to the front of the room.
• Give one student a set of unifix cubes and record that number on the board.
• Tell the student that you would like them to "take away" a given number of blocks and give them to the other student. Record this number. You should now have a subtraction problem that requires regrouping.
• Ask the class for volunteers to help the students figure out what to do.
• Use questioning to guide the students through the correct process (trade with the tens, cross out the number, write one less, add to the ones, cross out the number, add ten more, subtract ones, subtract tens).

Independent Practice

• Explain to students that they will be given unifix cubes and a partner. They should get a pencil and together do the four problems on the paper you are giving them.
• Use your method of deciding who goes first to decide who will hold the blocks the first time and who will be given blocks.

Closing

• In a math journal, tell with words or pictures how they would solve this problem: 312-147.

### Lesson Resources

 IP subtraction practice 1,947