GRADE 2 · ALBERTAOUTCOME AB-MAT2-N2
L6 · Missing numbers; checking with inverse operations
Learning goal
“I can find a missing number in a sum or a difference, and I can check an answer by using the inverse operation - addition and subtraction undo each other.”
Missing number — the number hidden by □. Our job is to find it.
Inverse operation — an operation that undoes another. Addition and subtraction are inverses.
Undo — to reverse what was done. Adding 27 is undone by subtracting 27.
Check — to prove an answer, usually by running the inverse operation.
Worked example
Find □ in 34 + □ = 61. Step 1 - COUNT ON: start at 34 and jump to 61: +6 lands on 40, +21 lands on 61. Step 2 - ADD THE JUMPS: 6 + 21 = 27. The jumps are the missing number: □ = 27. Step 3 - CHECK: 34 + 27 = 61. The missing number is 27.
Check yourself — explain it
How do you check that □ = 27 in 34 + □ = 61?
Paired problem — example still visible
□ + 25 = 58. Count on from 25 to 58. -> Jumps 5 and 28; □ = 33 (check: 33 + 25 = 58)
Faded completion — last step is yours
38 + □ = 75: 38 + 2 = 40, 40 + ___ = 75, so □ = ___ -> 35; 37
Ready to practise?
10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.