Finding the missing number
Learn how a number sentence works like a balance, and find the missing number with the opposite operation. Read one step, play with it, then tap “I understand this”.
= means: both sides are the same amount
A number sentence is like a balance. The = is in the middle, and both sides must weigh the same.
Make the balance level
Change the missing number until the balance hangs straight.
Plus and minus: do the opposite
To find the missing number, undo the sum with the opposite operation: the opposite of plus is minus.
? + 17 = 41 → 41 − 17 = 24
Always check: put your answer in the box and see if the sentence works. 24 + 17 = 41 ✓
Adding and subtracting undo each other, like opening and closing a door. In ? + 17 = 41, 17 was added; take the 17 away again and you are back at the start: 41 − 17 = 24.
Times and divide: opposites too
? × 6 = 42 → 42 ÷ 6 = 7
? ÷ 4 = 9 → 9 × 4 = 36
The undo machine
Choose a kind of sum. Try it yourself, then tap “Show me”.
Practise
Find the missing number
Type the number that goes in the box.
Word problems as number sentences
Write the story as a number sentence. The thing you do not know becomes the box.
Later in high school you write a letter instead of the box: x + 15 = 42. It is exactly the same idea!
Check yourself
- = means both sides are the same amount, like a level balance.
- Find the missing number with the opposite operation.
- Plus ↔ minus, times ↔ divide.
- Check: put your answer in the box and see if the sentence works.
Write these points in your maths book. Read them again the night before the test.
Quick questions
Tap an answer. If you get one wrong, read why and go back to that step. You get new questions every time.
You ticked all 5 steps! Now you are ready for the quiz.- Practice sheet for this classQuestions + memo · 2 A4 pagesPractice sheet
- Numbers, Operations and Patterns6 pages · PDFPaperMemo
- Patterns6 pages · PDFPaperMemo
Ready for more? The Grade 4 number sentences quiz gives you new sums every time.
- One step a day is enough. Ten calm minutes work better than an hour the night before the test.
- Play “I am thinking of a number”: “I think of a number, add 8 and get 20. What is my number?” That is exactly what this class practises.
- Ask your child to tell you the rule in their own words. If they can explain it, they understand it.
- The ticks (“I understand this”) are saved on this device only. We never ask for names or emails.
Follows CAPS for Grade 4 Maths. Always check your teacher’s notes and textbook too.