Second Grade · Math · week 14

Regrouping to subtract

Subtract 2-digit numbers by regrouping (unbundling a ten into ten ones) when the ones digit is too small.

Sometimes the top ones digit is smaller than the ones digit you need to subtract, like 32 - 17: you cannot take 7 away from 2. So you unbundle one ten from the tens place into 10 more ones.

After unbundling, the ones place has 10 extra to work with, and the tens place has one fewer ten. The total value of the number does not change -- you have just rearranged it.

📋 For the teacher
Teach it (10–15 min):
  • Use real blocks: physically break open a ten-rod into ten one-cubes so the "unbundling" is something they watch happen, not just a rule on paper.
  • Slow way down on problems like 70 - 34 (a zero in the ones place) -- they need to unbundle from the tens even though there is nothing showing in the ones place yet.
  • Have them check the subtraction by adding the answer back to the number subtracted.
  • Connect back to last week: this is the reverse move from the "carry a ten" step in addition.
Talk about it:
  • What does "unbundle a ten" mean, and why do you need to do it sometimes?
  • How is unbundling in subtraction the opposite of carrying in addition?
  • Why is 70 - 34 tricky when the ones place starts at 0?

Mastery looks like: They regroup correctly when subtracting and the ones digit is too small, including problems with a zero in the ones place.

Common stumbles: Subtracting the smaller digit from the larger regardless of position (flipping the ones); forgetting to reduce the tens place by one after unbundling.

See it visually

Interactive, spoken-aloud tools that show this idea in motion.

SubtractionTake away and count back, step by step.try →
Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Math for this learner.
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