Regrouping with Popsicle Sticks: Double-Digit Subtraction
Students will be able to perform two-digit subtraction with regrouping.
- Ask your students, "If you had a ten dollar bill and you needed to pay a friend back for the six dollars she lent you, what could you do?"
- Take student responses. If no one suggests breaking the ten dollar bill and exchanging it for smaller bills, then add suggest it.
- If you choose to regroup the ten dollar bill for smaller bills, how might that look? (You could break the ten dollar bill with 10 one dollar bills or a five and five ones.)
- Explain that understanding how to exchange a ten dollar bill for ten one dollar bills will help them with the maths strategy they will learn today.
Explicit Instruction/Teacher modeling(10 minutes)
- Write the number 43 on the surface being projected so that students can see. This can also be done on a rug with students sitting in a circle around the demonstration. Ask them what they know about this number. Note student responses, writing them down if you like.
- If it hasn’t been mentioned, explain that we know that the four represents four tens and the three represents three ones.
- Take four groups of ten popsicle sticks (or toothpicks) bundled with rubber bands and put them above the four. Explain that the sticks are going to represent the values in 43, putting four bundles of ten above the forty and three individual sticks above the number three.
- Now, write the number -29 under 43 so you have a subtraction problem. Proceed as if you were about to subtract the ones column, taking nine from three. Note that you can’t subtract nine from three, but that you CanTake 29 from 43 so we know there has to be a way.
- Ask students, "If we need more ones so that we can take nine away, how can we regroup the forty into ones?" Discuss. Have students think back to the ten dollar bill scenario.
- Take one bundle of ten and unbundle, or regroup it into the ones, adding it to the three that was already there.
- Note the new representation in the top number - when you add them they are still equal to 43, they are just grouped differently so that we can subtract.
- Subtract the ones place and then subtract the tens place using the standard subtraction process.
Guided practise(10 minutes)
- Do another example, this time on the board. Try using 61 - 45As the example.
- As you go through this example use the same process, except when you do the regrouping (cross out the six and write five, and add 10 ones to the amount in the ones place) slow down and remind students of the grouped popsicle sticks that the numbers represent and the exchange of a ten dollar bill for ten ones, connecting prior knowledge.
Independent working time(15 minutes)
- Distribute the Shark! Two Digit Subtraction with Regrouping worksheet.
- Do one or two problems together as a class on the board.
- Instruct students to complete the activity.
- Provide bundled groups of popsicle sticks to students who need to do more problems with the manipulatives.
- During Guided practise and Independent practise, do several more examples together as a class, gradually having students do more of the thinking.
Enrichment:Have students apply the skill to three-digit numbers and word problems with the Review Three-Digit Subtraction worksheet.
- Provide a few problems that require regrouping on the board. Have students turn their sheet over and work those problems. Collect or spot check for accuracy.
Review and closing(5 minutes)
- Write '$2, 8 quarters, and 200 pennies' on the board.
- Discuss, "How are they different, how are they the same? What are some real life examples of when we need to understand how the same values can represented differently?"