Lesson · Grade 3 · Mathematics
Finding the Missing Number
- Grade 3
- Mathematics
- 1 min 1 s
- British Columbia
- BC3-MA-19
The video shows that the equal sign means both sides are the same amount by turning 12 + □ = 20 into a row of 12 cubes with empty spaces above a row of 20 cubes. Counting on from 12 to 20 fills the 8 spaces, and 12 + 8 = 20 checks it; then the box moves to the start (□ + 15 = 20 makes 5) and the end (6 + 13 = □ makes 19), ending on "make both sides the same amount".
Concept: Finding the missing number in an equation
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In one sentence
The question this lesson answers
What number goes in the box?
The equal sign means both sides are the same amount.
Transcript
What the lesson says
What number goes in the box?
The equal sign means both sides are the same amount.
So twelve and the missing number must make twenty.
Start at twelve and count on to twenty.
Thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen, twenty.
We counted on eight, so the missing number is eight.
Check: twelve plus eight is twenty, so both sides match.
The box can be at the start, in the middle or at the end.
Fifteen and five make twenty, so this box is five.
Six plus thirteen is nineteen, so this box is nineteen.
To find the missing number, make both sides the same amount.
Curriculum
Where this fits
- British Columbia, Grade 3
Mathematics, Patterns and algebra: one-step addition and subtraction equations with an unknown numberBC3-MA-19
Previous: Writing a Pattern Rule: Start at 3, Add 2 Each Time
The outcomes and strands follow the British Columbia curriculum.
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