Unknown in Addition & Subtraction
PROCEDURALDetermine the unknown whole number in an addition or subtraction equation relating three whole numbers
Mastery Evidence
- Solve 8 + ? = 11 and write 3
- Solve 5 = □ − 3 and write 8
- Solve 6 + 6 = □ and write 12
Assessment Prompt
“If [child] sees '? + 6 = 14' or '9 − ? = 4' written on paper, can they figure out what number goes in the gap?”
Curriculum Standards1 alignment
1.OA.8Common Core State Standards for MathematicsDetermine the unknown whole number in an addition or subtraction equation relating three whole numbers. For example, determine the unknown number that makes the equation true in each of the equations 8 + ? = 11, 5 = [] – 3, 6 + 6 = [].
Prerequisites2
- Inverse: addition undoes subtractionsoftAges 6—7
- What the equals sign meanshardAges 6—7
Show full prerequisite tree
- Inverse: addition undoes subtraction soft
Solving missing-number problems benefits from knowing the inverse relationship
- Finding a missing number in addition hard
Inverse relationship builds on understanding subtraction as unknown-addend
- Addition as combining or putting together two hard
Unknown-addend requires understanding both addition and subtraction
- How Many in Total? hard
Understanding addition as combining groups requires knowing numbers represent quantities (cardinality)
- One-to-one counting hard
Cardinality principle builds on one-to-one correspondence — you must count correctly to know the last number tells 'how many'
- Subtraction as taking away or separating hard
Subtraction as unknown-addend reframes subtraction conceptually
- How Many in Total? hard
Understanding subtraction as taking away requires knowing numbers represent quantities (cardinality)
- One-to-one counting hard
Cardinality principle builds on one-to-one correspondence — you must count correctly to know the last number tells 'how many'
- Reading +, −, and = symbols hard
Deep understanding of = requires already being able to read and write number sentences
- Reading and writing numbers to 20 hard
Writing number sentences requires reading and writing numerals
- How Many in Total? hard
Reading/writing numerals 0–20 requires understanding that numerals represent quantities (cardinality)
- One-to-one counting hard
Cardinality principle builds on one-to-one correspondence — you must count correctly to know the last number tells 'how many'
- Writing digits 0-9 hard
Writing numerals requires the motor skill of forming digits 0-9 (taught in English handwriting)
- Addition as combining or putting together two hard
Reading/writing the + symbol requires understanding what addition means
- How Many in Total? hard
Understanding addition as combining groups requires knowing numbers represent quantities (cardinality)
- One-to-one counting hard
Cardinality principle builds on one-to-one correspondence — you must count correctly to know the last number tells 'how many'
- Subtraction as taking away or separating hard
Reading/writing the − symbol requires understanding what subtraction means
- How Many in Total? hard
Understanding subtraction as taking away requires knowing numbers represent quantities (cardinality)
- One-to-one counting hard
Cardinality principle builds on one-to-one correspondence — you must count correctly to know the last number tells 'how many'
Unlocks1
- Connecting maths to real lifesoftAges 6—7