There were 3 red, 2 blue, and 4 green. So 3 + 2 = 5.
For a problem with three numbers to add, the child writes an equation using only two of them and ignores the third, so the equation does not match the whole story.
The child has only practiced adding two numbers. When a third number shows up, their mental model still says 'an addition has two parts,' so they leave one number out.
Words rarely shift this one. It needs something to look at — a picture, a fold, an object on the table — before the explanation lands.
Puts the total in an addend spot
What you might hear
I got 9. So 9 + 2 + 4 = 3.
The child knows the answer to the whole story but writes it as one of the numbers being added instead of after the equals sign, so the equation is not true.
The child thinks the equals sign means 'the answer comes next' loosely, and grabs the big number they found and drops it into the first open spot they see.
This one responds to correction in the moment — the idea is there, the rule being applied is not.
Thinks it needs two equations in one fixed order
What you might hear
No, you HAVE to do 8 plus 7 first, then add 2. You can't start with the 8 and 2.
The child believes a three-number story can only be solved by adding the first two, then the third, and refuses any other grouping, so they get stuck or wrong when the first two are hard.
The child learned 'add two at a time' as a rigid rule and does not yet see that the three numbers can be grouped in any order to make adding easier.
This one is not a slip to correct — the underlying idea needs re-teaching before more practice will help.
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.
On three days Akira collected 8, 2, and 7 shells. Which equation tells how many Akira collected altogether?
A8 + 2 = 10
B8 + 2 + 7 = 17
C8 + 2 - 7 = 3
D8 + 2 + 7 = 19
AnswerB — 8 + 2 + 7 = 17
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 "Represent three-addend problems with equations" 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
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