Maths

My Child Calculates Correctly But Picks the Wrong Operation in Word Problems

13.08.2026 · 6 min read

When an 8-year-old solves 24 + 6 instead of 24 − 6, the problem isn't the arithmetic — it's that the text never turned into a picture in their mind. Here's how to check for this and fix it at home.

Martin, in second grade, reads: "Vanya had 24 stickers. She gave 6 to her brother. How many does she have left?" He writes 24 + 6 = 30. Five minutes earlier, this same Martin solved 24 − 6 in his head without a single mistake. The parent sighs, "You're not reading carefully!" And that's where the real misunderstanding happens — the child read carefully. He just read for words, not for the story.

In primary school, children quickly pick up a shortcut: the text contains signal words. "Altogether" means addition, "left" means subtraction, "each" means multiplication. This shortcut works in roughly half of all problems, and that's exactly why it's dangerous — it gets reinforced often enough to feel like a rule. As soon as a problem is phrased a bit differently ("Vanya gave away 6 stickers and had 18 left. How many did she start with?"), the shortcut leads the child in the wrong direction, and the calculation that follows is flawlessly correct — and flawlessly wrong.

How to tell whether the problem is with the maths or with the understanding

There's a quick check that takes less than a minute. Give the same problem but with smaller numbers: instead of 24 and 6, use 5 and 2. If the child now picks the right operation, the calculation itself was the load that was crowding out the meaning. If they still get the direction wrong with 5 and 2, they never actually pictured the situation — the problem lies in translating text into a mental image.

• Solves correctly with small numbers, gets it wrong with big ones → the thinking works, but the numbers overload working memory. Practise on scrap paper with lighter numbers until the pattern sticks.
• Gets it wrong even with small numbers → the child isn't picturing the story. Only acting it out with objects or a drawing helps here.
• Reads the problem and immediately writes a sign, with no pause → the keyword autopilot has kicked in.
• Asks "is it plus or minus?" → waiting for permission from outside instead of checking for themselves whether the answer makes sense.

The question that changes everything

Before any calculating, ask: "In the end, is there more or less?" Just that. The child isn't allowed to write a single digit until they answer. "Less" automatically rules out addition and multiplication — and that's half the battle won. It sounds too simple, but it's exactly the step missing from most exercise books, where children jump straight to the sum.

If there's less at the end, then I can't have been adding. So my calculation is correct, but the operation isn't.

The second good question is: "Whose was the sticker at the start, and whose is it at the end?" Word problems almost always describe movement — something goes somewhere, comes from somewhere, or gets split up. Once the child can trace that movement with a finger across the table, the correct sign appears on its own.

The most common parenting mistake

The most popular rescue attempt is a list of keywords stuck on the fridge: "altogether = +", "left = −". It looks practical, but in practice it doubles the problem. The child gets official permission not to read the problem at all, just to scan it for words. By third and fourth grade, when problems become two-step and start containing "altogether" in a sentence that actually calls for subtraction, the list turns against them. Worse still — by then the child has already lost the habit of picturing the story, and that habit is far harder to rebuild than it was to form in the first place.

The other common trap: the parent reads the problem aloud instead of the child, with just the right intonation, unintentionally stressing the key word. The child answers correctly and everyone's pleased. On the test, though, nobody reads it with intonation for them.

The check that should become a habit

Once the answer is ready, the child puts it back into the sentence: "Vanya has 30 stickers left." Then asks one single thing: "Can she have more left than she started with?" This check takes five seconds and catches almost every mistake of this kind. At first, do it out loud yourself so the child hears what it sounds like. After two or three weeks, hand it over to them — and don't jump in to correct them; wait for them to ask themselves the question.

• Read the problem twice: the first time for the story, the second for the numbers.
• Say in one word whether there ends up being more or less.
• Draw a picture or act it out with objects if you're not sure.
• Write the sum.
• Put the answer back into the sentence and check that it sounds possible.

Five steps, and only the fourth one is calculating. That's exactly why children who are excellent at arithmetic still get it wrong — they've only ever practised one-fifth of the job.

How much time this takes

No extra hours needed. Two problems a day, worked through properly using all five steps, beat ten problems rushed through. If a child solves two problems and, for both, simply states whether the answer is more or less before writing anything — that's more real progress on understanding than a whole page of completed sums. One more thing: do this on days without a test. Under pressure, every child reverts to the keyword shortcut.