Why does the ones digit stay the same when you add ten?

Because ten is a whole bundle of ten ones, so it joins the tens and never disturbs the loose ones. Thirty-four is three bundles and four loose, and adding a bundle leaves the four loose ones untouched. Saying it in bundles once or twice usually does more than repeating the rule, because the rule then describes something your child can see.

Is it a problem if my child counts on ten instead of jumping?

Not at first, but it is the thing to move past, because counting is where errors and slowness both come from. Treat counting as a sign the ten is still ten separate ones in their head rather than one object. A hundred square or a bead string makes the single jump visible, and most children switch over within a few sessions.

When does this skill matter, and what does it lead to?

It arrives in first grade and is expected to be instant by second, because almost every mental addition strategy for larger numbers moves in tens first and deals with the ones afterwards. A child adding 47 and 30 jumps three tens and is done. The same idea extends to adding and subtracting a hundred, where only the hundreds digit moves.

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Ten More, Ten Less

Jump ten in one move.

Grades 1-3 · Adding and subtracting ten without counting

How to play

1A number appears with an instruction: ten more, or ten less.
2Tap the answer.
3Only the tens digit changes. The ones digit stays exactly where it is.
4Keep your streak alive.

Try a few

34, ten more
Answer: 44
Say the two digits separately: three tens and four ones. Adding ten adds one more ten and touches nothing else, so it becomes four tens and four ones, which is 44. The four on the end never moves, and noticing that it never moves is the whole lesson. A child who has to count from thirty-four up to forty-four has not yet seen that ten is a single object you can hand over in one piece.
12, ten less
Answer: 2
Twelve is one ten and two ones. Take the ten away and the one ten is gone, leaving just the two ones, so the answer is 2. This is the case that catches children out, because the tens digit does not merely shrink, it disappears from how the number is written. Nothing strange has happened: the ones digit stayed at two, exactly as it does every time, and there is simply no ten left to write down in front of it.
87, ten more
Answer: 97
Eight tens and seven ones, plus one more ten, gives nine tens and seven ones, which is 97. It is worth doing this one aloud because eighty-seven feels large enough that children want to reach for counting, and counting on ten from eighty-seven is ten chances to slip. The digit move is instant and the counting is not, which is the argument for the digit move. The numbers in this game stop below a hundred, so the tens digit always has room to grow by one.

Why this game helps

Ten more than 34 should not require counting. It is 44, because adding ten changes the tens digit and leaves the ones alone, and a child who sees that has understood something real about place value rather than memorized a result. The tell that it has not landed is the answer 35: the child heard "more" and added one, because every number move so far has been a single step. Practising the ten-jump until it is instant is what makes mental addition of larger numbers possible, since almost every strategy for those runs through jumping by tens first.

If your child finds this hard

The classic wrong answer is 35 for ten more than 34. The child heard the word more and did the only number move they own, which is a single step forward, and this is the slip recorded most often for adding ten. There is a useful detail here: the answers offered in this game are always within two of the correct one, so 35 can never appear as a choice. A child who adds one will hunt for their answer, fail to find it, and have to read the question again, which is a better correction than being told.

The second error is changing the wrong digit, turning ten less than 62 into 61 or into 72. This is a child treating the two digits as a pair of separate small numbers rather than as tens and ones, so it looks arbitrary which one gets adjusted. Have them say the number in expanded form before answering, six tens and two ones, then say what ten less does to that sentence. Once the sentence is right, the digits follow without a decision.

A hundred square settles this faster than any explanation. Print one, or draw ten rows of ten, and let your child put a counter on 34 and another on 44. Ten more is always the square directly below, ten less the square directly above, and the column never changes because the ones digit never changes. After a dozen jumps down and up the column, the rule stops being something to remember and becomes a direction on a grid your child can picture with their eyes shut.

Common questions

Why does the ones digit stay the same when you add ten?: Because ten is a whole bundle of ten ones, so it joins the tens and never disturbs the loose ones. Thirty-four is three bundles and four loose, and adding a bundle leaves the four loose ones untouched. Saying it in bundles once or twice usually does more than repeating the rule, because the rule then describes something your child can see.
Is it a problem if my child counts on ten instead of jumping?: Not at first, but it is the thing to move past, because counting is where errors and slowness both come from. Treat counting as a sign the ten is still ten separate ones in their head rather than one object. A hundred square or a bead string makes the single jump visible, and most children switch over within a few sessions.
When does this skill matter, and what does it lead to?: It arrives in first grade and is expected to be instant by second, because almost every mental addition strategy for larger numbers moves in tens first and deals with the ones afterwards. A child adding 47 and 30 jumps three tens and is done. The same idea extends to adding and subtracting a hundred, where only the hundreds digit moves.

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