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.
Worked examples
Why is it allowed?
- Why can 56 + 19 become 55 + 20?
- Move some from one number to the other.
- The total stays the same, so the step is fair.
- So the answer is Moving some from one number to the other keeps the total..
Check a take-away
- Can 49 − 18 become 47 − 20?
- Take-aways must change both numbers equally.
- Compare the changes.
- So the answer is Not allowed.
Check a bigger swap
- Can 168 + 397 become 165 + 400?
- Add up what moved.
- If the total is the same, it's allowed.
- So the answer is Allowed.
A taste of the worksheet
These are the first problems of set A. The printable sheet has many more, with an answer key.
- Why can 35 + 38 become 38 + 35__
- Why can 21 + 48 become 48 + 21__
- 67 + 19 = 70 + 16. Why can 70 + 16 become 80 + 6__