Grade 2 · Operations · MAT-02-OPS-002
Subtraction Facts Within 20 Fluency
Recall or efficiently derive subtraction facts within 20.
Learning objectives
What you should be able to do
- Recall or efficiently derive subtraction facts within 20.
- Use addition relationships, make-ten, and counting-back strategies.
- Explain subtraction as missing-addend and take-away reasoning.
Prerequisite check
Make sure the foundation is ready.
Checks the inverse-operation foundation.
Learn
Build the idea from meaning, not memorization.
Subtraction facts are linked to addition facts
If 8+7=15, then 15−8=7 and 15−7=8. This fact-family relationship lets known addition facts support subtraction fluency.
Think missing addend
For 14−6, ask '6 plus what equals 14?' This often uses addition fluency more efficiently than counting backward one by one.
Make ten supports subtraction too
For 13−5, first subtract 3 to reach 10, then subtract the remaining 2: 13−3−2=8.
Choose a strategy that matches the fact
Subtracting 1 may be best as one-less reasoning, while 17−9 may be easier as a missing-addend problem. Fluency means flexible strategy selection.
Interactive math lab
Change it. See what stays true.
Number Line Explorer
Move along the number line.
4 is one less. 5 and 6 is one more.
- count back
- missing addend
- make ten
- fact family
Worked examples
Twenty different ways to see the concept work.
15−8
7Because 8+7=15.
15−7
8Inverse of 7+8=15.
14−6
86+8=14.
13−5
813−3=10, then −2=8.
12−7
57+5=12.
18−9
9Double 9 gives 18.
16−8
8Double 8 gives 16.
17−8
98+9=17.
11−3
83+8=11.
10−7
37+3=10.
9−1
8One less.
20−2
18Subtract two ones.
19−9
109+10=19.
14−7
7Double 7 inverse.
13−6
76+7=13.
12−4
84+8=12.
11−8
38+3=11.
18−7
117+11=18.
20−10
10Ten removed from twenty.
16−9
79+7=16.
Practice
Guided practice with hints
0114−5
Hint: Ask 5+?=14.
Answer: 9
0213−8
Hint: Ask 8+?=13.
Answer: 5
0317−9
Hint: Use 9+?=17.
Answer: 8
0412−2
Hint: Take away two.
Answer: 10
0518−8
Hint: Use 8+10.
Answer: 10
0615−6
Hint: Use fact family.
Answer: 9
Practice
Independent practice
0115−7
Answer: 8
0216−9
Answer: 7
0313−4
Answer: 9
0412−6
Answer: 6
0518−9
Answer: 9
0611−8
Answer: 3
0720−5
Answer: 15
0814−8
Answer: 6
Common mistakes
Learn to catch the error, not just the answer.
Use addition facts and missing-addend reasoning when more efficient.
15−7 is not the same as 7−15 in whole-number Grade 2 contexts.
Decompose the subtrahend explicitly.
Applications & challenge
Use the idea beyond a single exercise.
Applications
- Mentally find how many items remain after small take-away situations.
- Use subtraction facts to check addition totals.
Challenge problems
Find two ways to solve 16−7.
For example missing addend 7+9=16 or subtract 6 to 10 then 1.
Why is subtraction not commutative?
Changing order changes the meaning and result.
Which addition fact helps 18−11?
11+7=18
Explain why 13−5 can be 13−3−2.
5 is decomposed as 3+2; subtracting both parts removes 5 total.
Flashcards
Retrieve it from memory.
Flashcards
Card 1 of 6
Game mode
Fact Family Rescue
Use known addition facts to solve subtraction facts within 20.
Practice
Mastery check
0115−6
Answer: 9
0217−8
Answer: 9
0312−7
Answer: 5
0418−9
Answer: 9
0514−4
Answer: 10
0620−8
Answer: 12
07Name one efficient subtraction strategy.
Answer: Missing addend, make ten, or fact family.
08Explain how addition checks subtraction.
Answer: The difference plus the removed amount should equal the starting amount.
Terminology
Words to know
- difference
- The result of subtraction.
- missing addend
- An unknown part in an addition relationship.
- fact family
- Related addition and subtraction facts using the same numbers.
- inverse operations
- Operations that undo one another.
Continue learning
Connected concepts
Curriculum references
Standards and instructional references
California Common Core State Standards for Mathematics — 2.OA.B.2
Fluency with subtraction within 20.