"Three plus four is seven, so the answer is seven!" (the third number, 2, is left out)
When adding three numbers, the child adds the first two and stops, leaving the third number out of the sum.
The child has only ever added two numbers before. Three numbers feels new, so they fall back to the old habit and treat the last number like it is not there.
This one responds to correction in the moment — the idea is there, the rule being applied is not.
Counts a number twice when making ten
What you might hear
"Six and four make ten, then ten and four more is fourteen." (the 4 was already used to make ten)
While grouping two numbers to make ten, the child reuses one of those numbers a second time when adding the third, so it gets counted twice.
Making ten means looking at the same numbers more than once. The child loses track of which number was already used and counts it again.
Words rarely shift this one. It needs something to look at — a picture, a fold, an object on the table — before the explanation lands.
Thinks the answer must stop at 20
What you might hear
"Nine and eight and five... I can only go to twenty, so it's twenty!"
The child hears "sum within 20" and treats 20 as a wall. When the three numbers add up past 20, they just answer 20 instead of the real total.
The child mixes up the rule with the answer. They think "within 20" means the answer is 20, so they stop counting once they reach it.
This one responds to correction in the moment — the idea is there, the rule being applied is not.
What to say when you see it
For each mistake above, Algo School gives you the words to say next — the specific response that corrects the thinking instead of just marking the answer wrong, plus practice aimed at that exact error.
Generated by add-sub-word-problems-1, a deterministic question engine — not written by an AI, and verified before any child sees it.
FAQ
Questions parents ask about this
How do I know which of these mistakes my child is making?
Look at what they SAY, not just what they wrote. Each mistake above includes the phrasing a parent typically hears — that sentence is usually the giveaway. In Algo School, the tutor recognises these patterns while your child is working and adapts the session around the one it sees.
Is "Add three whole numbers with sum within 20" something children normally struggle with?
Yes. We track 3 distinct approved mistake patterns for this concept, each one written up because it recurs — not because it is unusual. A child making one of them is doing something systematic, which is far easier to fix than random errors.
What should I do once I know which mistake it is?
Respond to the thinking, not the answer. Algo School gives parents the specific wording for each mistake, then generates practice targeted at that error and shows you whether it actually cleared.
Where do these mistakes come from?
They are part of the same content library that drives practice: each one is reviewed and approved before it is used, and the Operations and Algebraic Thinking questions on this site are checked by an automated verification pass before a child is ever served them.
Related
Other places children go wrong
Where this sits
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