'A hundred is just ten. You count one, two, three... ten, and that's a hundred.'
Child says 100 is made of ten ones (ten single cubes) instead of ten tens (ten bundles of ten).
The child remembers that ten ones make a ten and copies the same pattern, expecting ten of the next thing to make a hundred — but skips that the next thing is a whole ten, not a one.
Words rarely shift this one. It needs something to look at — a picture, a fold, an object on the table — before the explanation lands.
Says 100 has ten tens AND ten ones at once
What you might hear
'There are ten tens, and then there's ten ones too, so it's more than a hundred.'
Asked how many tens are in 100, the child gives an answer like 'ten tens and ten ones' or '110,' double-counting the same blocks.
The child has not learned that the same hundred can be NAMED in different ways — ten tens OR one hundred — so they think they must add up every label they know instead of seeing them as the same amount.
This one responds to correction in the moment — the idea is there, the rule being applied is not.
Counts the bundles as ones
What you might hear
'I count them — one, two, three... there are ten. So it's ten.'
Shown ten ten-bundles, the child counts 'one, two, three...' up to ten and says the total is ten, ignoring that each bundle is worth ten.
The child counts the number of OBJECTS in front of them (ten things) and forgets that each object stands for ten, so the bundle's value is lost.
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.
Look at the picture. Each flat is one hundred. How many tens make the same amount?
Answer40.0
Generated by three-digit-place-value, 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 "Understand 100 as a bundle of ten tens" 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 Number and Operations in Base Ten 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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