Why can my child add, but not solve missing-number questions like ? + 5 = 12?
By the BrightHop team · Published · Updated
Short answer
Adding 7 + 5 asks a child to join two parts. A missing-number question such as ? + 5 = 12 asks them to work out a part when they know the whole and the other part. That is a different idea, built on knowing that parts make a whole. A child can have the first without the second yet, and the fix is to teach the part-part-whole relationship, not to practise more joining.
Why this happens
In 7 + 5 = ?, the numbers appear in the order they are used, and the answer comes after the equals sign. In ? + 5 = 12, the child must read the equals sign as 'is the same as', hold the total in mind, and work backwards. Many children first meet the equals sign as 'the answer comes next', so the unknown at the start looks wrong to them.
The idea underneath is part-part-whole: two parts make a whole, so if you know the whole and one part, the other part is what is left. The Australian Curriculum includes part-part-whole knowledge to 10 in Year 1 addition and subtraction, and number sentences in Year 2.
Example: three ways it goes wrong
- Adds the two numbers shown: 6 + 16 = 22. The child has treated the question as 'join'.
- Counts on from 6 but counts 6 as the first jump, so lands one short or long.
- Gives 6 again, repeating the part they can see.
BrightHop's missing-number lesson records the first two as named mistakes: 'adds the two numbers instead of finding the missing part' and 'counts the starting number as one of the jumps'. They need different fixes, which is why it helps to see the wrong answer, not just that it was wrong.
What parents can do
- Use objects first. Put 12 counters in a row, cover 7 and ask how many are hidden.
- Say it as a story: 'There were some biscuits, then 5 more, and now there are 12. How many were there at the start?'
- Teach counting on: start on the part you know, count the jumps to the total. Start on the number, then count the jumps after it.
- Check the answer by adding it back: does 7 + 5 give 12?
- Only then write the number sentence with the gap in different places: 7 + ? = 12, ? + 5 = 12.
How to know whether this is the real gap
If the child cannot do 7 + 5 either, the gap is earlier. If they hesitate on 7 + 3 = 10 or ? + 3 = 10, look at number bonds to 10, which the missing-number idea relies on.
What to teach next
| Before | This skill | After |
|---|---|---|
| Number bonds to 10 and simple addition | Finding the missing number, first with the total unknown, then a part unknown, then the first part unknown | Fact families (addition and subtraction linked) in Year 2 |
Try it
Not sure whether this is the gap your child has? The free addition Skill Check includes missing-number questions and shows which part to work on. No account needed.
Try the free addition Skill CheckFrequently asked questions
Is ? + 5 = 12 harder than 7 + 5 = ?
Usually, yes, for Year 1 and Year 2 children. The calculation is the same, but the child must understand how the parts and the whole relate, and read the equals sign as 'is the same as'.
Should I let my child count on their fingers?
Yes, at this stage. Counting on is a legitimate strategy and the way to understand why the answer is right. Speed comes later from knowing the number bonds.
Related guides
Sources
This guide is written by the BrightHop team from BrightHop’s own teaching approach. It is not independent research and not official curriculum guidance. Curriculum and research statements link to their original sources above. Skill Checks are informal and do not replace a teacher’s advice. About BrightHop