Chapter overview



Missing number problems

Your lesson progress



Missing number problems

​​In a nutshell

Expressions involving an unknown number are needed to solve missing number problems. Unknown numbers are usually represented by letters.

Example 1

x+10=12x + 10 = 12

This is the same as asking: "what is added to 1010​ to get 1212​?"

The answer is x=2x=2

Note: xx is the letter commonly used for an unknown.

Method for solving


Write out an equation 
Work out the value of the unknown

Example 2

Anna has £20£20​. She shares £5£5​ with her brother and uses £12£12​ to buy a new dress. how much money does she have left?

Write an expression:

5+12+x=205 + 12 + x = 20

Solve for xx:

x=3x= 3

Anna has £3\underline{£3}​ remaining.

Frequently Asked Questions (FAQ)


  • Question: How do I solve a missing number problem?

    Answer: Use trial and error to try out possible solutions.

  • Question: What do letters mean in an unknown number problem?

    Answer: Unknown numbers are usually represented by letters.



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