Concept 6 of 6
Missing Numbers & Adding Three
Today's big idea
Subtract the part you know from the total to find a gap.
7 + ? = 15 means 15 − 7 = 8.
How it works
👀 What's going on?
7 + ? = 15. What number is missing?
💡 Here's the trick
To find a missing part, subtract the part you know from the total. To add three numbers, add two first, then the third.
7 + ? = 15 means 15 − 7 = 8. And 4 + 3 + 6: first 4 + 3 = 7, then 7 + 6 = 13.
✍️ Try it
9 + ? = 16 means 16 − 9 = 7.
⚠️ Watch out
With three numbers, add two at a time, never all at once.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“8 + ? = 14. What is missing?”
- 1
Find the missing part
Subtract the part you know from the total.
- 2
Do the subtraction
14 − 8 = 6
- 3
Answer ✓
? = 6
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Subtract the part you know from the total.
8 + ? = 14
14 − 8 = ?
Quick check
✅ VERIFICATION
What is 5 + 4 + 6?
⚡ Finish the activity above first to answer.
Keep going!