Grade 1 · Operations · MAT-01-OPS-008
Subtract Within 20
Subtract within 20 using think-addition, make-ten, count-back, and decomposition strategies.
Learning objectives
What you should be able to do
- Subtract within 20 using think-addition, make-ten, count-back, and decomposition strategies.
- Choose a strategy that matches the relationship between the numbers.
- Explain subtraction across ten using place-value and part-whole reasoning.
Prerequisite check
Make sure the foundation is ready.
Checks basic subtraction fluency.
Checks inverse reasoning.
Learn
Build the idea from meaning, not memorization.
Think-addition is powerful for close numbers
For 15−13, ask 13+?=15. Because only two are missing, the difference is 2. This is often easier than counting backward thirteen steps.
Make-ten can support subtraction across ten
For 14−6, split six into four and two. First 14−4=10, then 10−2=8. Moving through ten uses a familiar benchmark.
Decomposition keeps quantities organized
Breaking the subtracted amount into useful parts does not change the total removed. The parts still add to the original subtrahend.
Addition checks subtraction
If 16−7=9, verify with 9+7=16. The remainder and removed part should reconstruct the whole.
Interactive math lab
Change it. See what stays true.
Equation Balance
Adjust both sides. Equality means both expressions have the same value.
Balanced: both sides have equal value.
- think addition
- bridge through ten
- fact family
- check with addition
Worked examples
Twenty different ways to see the concept work.
11−2
9Count back two or think 9+2=11.
12−3
93+9=12.
13−4
9Subtract 3 to reach 10, then 1 more.
14−5
9Subtract 4 to reach 10, then 1.
15−6
9Subtract 5 to reach 10, then 1.
16−7
9Subtract 6 to reach 10, then 1.
17−8
9Think 8+9=17.
18−9
99+9=18.
19−9
10Remove nine ones from ten-plus-nine.
20−10
10Two tens minus one ten leaves one ten.
14−6
814−4=10, then −2=8.
13−5
813−3=10, then −2=8.
12−7
5Think 7+5=12.
15−7
8Think 7+8=15.
16−8
8Double relationship 8+8=16.
17−15
2Think 15+2=17.
19−18
1Think 18+1=19.
10−4
6Basic fact within 10.
Explain 14−8 with make-ten.
14−4−4=6Split eight into four and four.
Explain 18−11 using think-addition.
11+7=18, so 7Find the missing part.
Practice
Guided practice with hints
01Solve 13−4.
Hint: Reach ten first.
Answer: 9
02Solve 14−6.
Hint: Subtract 4, then 2.
Answer: 8
03Solve 15−7.
Hint: Ask 7+?=15.
Answer: 8
04Solve 16−8.
Hint: Use a double.
Answer: 8
05Solve 19−18.
Hint: Think addition.
Answer: 1
06Check 17−9=8.
Hint: Use addition.
Answer: 8+9=17
Practice
Independent practice
0112−4
Answer: 8
0213−6
Answer: 7
0314−7
Answer: 7
0415−8
Answer: 7
0516−9
Answer: 7
0618−8
Answer: 10
0717−14
Answer: 3
0820−9
Answer: 11
Common mistakes
Learn to catch the error, not just the answer.
Use think-addition for cases such as 18−16.
Check the decomposition before subtracting.
Label the first part that reaches ten, then subtract the leftover part.
Applications & challenge
Use the idea beyond a single exercise.
Applications
- Find how many items remain after using part of a group within 20.
- Find a missing difference between two small counts or scores.
Challenge problems
Solve 18−7 through ten.
18−8 would hit 10, but subtract only 7: one efficient path is 18−7=11; or think 7+11=18.
Why is 17−16 best by think-addition?
Only one more is needed from 16 to 17.
Compare 14−6 strategies.
Make-ten: 14−4−2=8; think-addition: 6+8=14.
How does place value help with 19−9?
19 is one ten and nine ones; removing nine ones leaves one ten.
Flashcards
Retrieve it from memory.
Flashcards
Card 1 of 6
Game mode
Strategy Sprint: Subtract 20
Choose think-addition, bridge-through-ten, or a fact family to subtract within 20.
Practice
Mastery check
0113−5
Answer: 8
0214−6
Answer: 8
0315−7
Answer: 8
0416−8
Answer: 8
0517−15
Answer: 2
0619−9
Answer: 10
07Name a strategy for 18−16.
Answer: Think addition.
08Check 14−6=8.
Answer: 8+6=14
Terminology
Words to know
- difference
- The result of subtraction.
- think addition
- Solve subtraction by finding the missing addend.
- bridge through ten
- Use ten as an intermediate benchmark during subtraction.
- decompose
- Split a quantity into helpful parts.
Continue learning
Connected concepts
Curriculum references
Standards and instructional references
California Common Core State Standards for Mathematics — Grade 1 Operations & Algebraic Thinking
Grade 1 addition/subtraction within 20, word problems, and equations with unknowns.