Understand the idea.
A box stands for a number
A box stands for a missing number, and the equals sign says both sides have the same value. In , nine is one part of fourteen. Count from nine to fourteen, or subtract, to find the other part.
Read the whole equation
Notice what the box represents before choosing a calculation. In , it is the starting whole. Join the seven removed and the twelve left over. Then put your answer back into the original equation to test it.
See the method in action.
Missing added part
Find the missing number: 8 + □ = 17.
Worked solution
- 1
Count from eight to ten.
The missing added part must cover the distance from eight to seventeen.
- 2
Continue from ten to seventeen.
The remaining distance after the first jump is seven.
- 3
Combine the jumps and check.
Nine fills the box because the two sides then have equal value.
9
Missing starting amount
Find the missing number: □ − 6 = 18.
Worked solution
- 1
Identify the two parts of the missing whole.
The starting amount contains both the eighteen left and the six removed.
- 2
Put twenty-four back into the equation.
Subtracting six leaves the required eighteen, so the box is twenty-four.
24
Try it yourself.
Check your understanding
Find □ in 25 − □ = 20.
Show answer & reasoning
5
The missing number is the part removed from twenty-five. Since and , five fills the box.
Keep practising
6 worksheet questions
Read the questions online
- Complete: .
- Complete: .
- Complete: .
- Complete: .
- Complete: .
- Maya chooses 13 for . Show a check.
Read the worked solutions
- Complete: .
.
5 - Complete: .
.
7 - Complete: .
.
8 - Complete: .
.
27 - Complete: .
Count on 6 from 34 to 40.
6 - Maya chooses 13 for . Show a check.
Substitute the chosen number into the original equation.
, so 13 works.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Facts within 20; one-/two-digit calculations; partitioning; related facts; missing numbers; checking.
Official curriculum source ↗