3-Digit Subtraction · Regroup in Consecutive Places · Horizontal

Teaching Notes

Focus: Three-Digit Subtraction: Regroup in Consecutive Places. Use the worked example and teaching sequence below to explain the reasoning before independent practice.

Before You Begin

• Read the numbers and identify the operation.

Teaching Steps for Parents and Teachers

1. Start in the ones place and subtract corresponding places.
2. If a top digit is too small, exchange one unit from the next place for ten units in the current place.
3. Mark both changes: the giving place decreases by one and the receiving place increases by ten.
4. Continue left, using the changed digits.

Worked Examples

Evaluate 632-278.

1. Regroup from place value 10; it decreases by 1. There are now 12 units at place value 1.
2. Place value 1: 12 − 8 = 4.
3. Regroup from place value 100; it decreases by 1. There are now 12 units at place value 10.
4. Place value 10: 12 − 7 = 5.
5. Place value 100: 5 − 2 = 3.

Answer: 354

Common Mistakes and How to Help

Do not reverse the digits just because the lower digit is larger. Subtraction keeps its original order. When a zero is involved, follow the exchange across the places rather than changing every zero independently.

Add the difference to the number subtracted. You should recover the starting number.

Questions to Check Understanding

• Add the difference to the number subtracted. You should recover the starting number.
• Which step explains your answer?
• Can you apply the same reasoning when the numbers change?

Practice Advice

Work through the example together. Ask the learner to explain the first two problems, then try three independently. Review the first incorrect step before assigning more practice.

Next Practice

• Use New Numbers to practice the same skill with different values.
• If the learner needs help, return to the example and model each step before trying again.