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Grade 2 · Maths · Lessons
Why that step is allowed
🔄swap⚖️same total✅allowed🚫not allowed
Fair swaps.
The big ideaA step is allowed when the amount stays the same.

How it works

A step is allowed when the amount stays the same.

For adding, you can move some from one number to the other: 56 + 19 = 55 + 20.

For taking away, you must change both numbers by the same amount.

Allowed• 56 + 19 = 55 + 20• total stays the sameNot allowed• 49 − 18 → 47 − 20• the difference changes
A fair swap and an unfair one.

Example 1: Why is it allowed?

  1. Why can 56 + 19 become 55 + 20?
  2. Move some from one number to the other.
  3. The total stays the same, so the step is fair.
  4. So the answer is Moving some from one number to the other keeps the total..

Example 2: Check a take-away

  1. Can 49 − 18 become 47 − 20?
  2. Take-aways must change both numbers equally.
  3. Compare the changes.
  4. So the answer is Not allowed.

Example 3: Check a bigger swap

  1. Can 168 + 397 become 165 + 400?
  2. Add up what moved.
  3. If the total is the same, it's allowed.
  4. So the answer is Allowed.

Words to know

allowed
A step that keeps the amount the same.
total
The amount you get when you add.

Watch out

Try it

  1. 19 + 16 = 20 + 15. Can 20 + 15 become 20 + 5?
  2. Can 22 + 49 become 22 + 50 + 1?
  3. Can 82 − 43 become 82 − 40 − 3?
Answers: 1. Not allowed · 2. Not allowed · 3. Allowed
For grown-upsAsk 'does it still add to the same?' when your child changes a sum.
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