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❓ Finding the Missing Number

Find the mystery number in equations like 8 + ? = 11, using what you know about how addition and subtraction fit together.

lessons

~15 min

to learn

🔢 Math

subject

Ages 6–12

level

Start the course →

What you’ll learn

The Unknown in a SumFind an unknown in an addition equation, like 8 + ? = 11, by counting on or subtracting.To find a missing number in a sum, count on from the known number to the total, or subtract to undo the addition. For 8 + ? = 11, count 9, 10, 11 (three hops) or compute 11 - 8 = 3. Always check by putting the number back.
Unknowns in Take-AwaysFind an unknown in a subtraction equation, like 5 = ? - 3, by undoing the operation and checking.The unknown can appear anywhere. For 5 = ? - 3, add the 3 back: 5 + 3 = 8. For 14 - ? = 9, count back to find the gap: 5. Whatever the puzzle, undo the operation and then check by putting the answer back into the sentence.

Questions this course answers

What is the missing number: 8 + ? = 11?

Count on from 8 to 11: 9, 10, 11 — that is 3 hops.

Solve: 5 = ? - 3.

Add the 3 back: 5 + 3 = 8. Check: 8 - 3 = 5.

Grounded in trusted sources

Khan Academy Kids
PBS Kids — early math
National Council of Teachers of Mathematics (NCTM)

Every Wunder lesson is built from real, reputable sources — never invented.

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