Common Mistakes
Learn to spot and avoid errors that many students make
📖 Learn
Everyone makes mistakes when learning math - that's how we learn! Let's look at common mistakes and learn how to avoid them.
Mistake #1: Counting the Starting Number
When counting on, some students accidentally count the starting number.
Wrong Way
Counting: "8, 9, 10" = 10
The student counted 8 as one of the 3!
Right Way
Start AT 8, then count 3 MORE: "9, 10, 11" = 11
Tip
Say "Start at 8" then count UP: "9, 10, 11"
Mistake #2: Adding Instead of Subtracting
Some students add when they should subtract (or the other way around).
Wrong Way
The student added instead of subtracted!
Right Way
The minus sign (-) means take away, so the answer is SMALLER.
Tip
Check: When you subtract, your answer should be smaller than the first number!
Mistake #3: Counting Back Wrong
When subtracting by counting back, some students count wrong.
Wrong Way
Counting back: "12, 11, 10, 9" = 9
The student counted 12 as one of the 4!
Right Way
Start AT 12, count back 4: "11, 10, 9, 8" = 8
Tip
Use your fingers! Put up 4 fingers, then count back one finger at a time.
Mistake #4: Mixing Up Numbers
Sometimes students write numbers backward or in the wrong order.
Wrong Way
Right Way
Tip
For teen numbers, the 1 comes FIRST (it means "ten"). So thirteen = 13, not 31.
Mistake #5: Forgetting to Check Your Work
Many mistakes happen because students rush and don't double-check.
How to Check Addition
Use subtraction! If 8 + 5 = 13, then 13 - 5 should equal 8.
How to Check Subtraction
Use addition! If 14 - 6 = 8, then 8 + 6 should equal 14.
💡 Examples
Let's practice spotting and fixing mistakes!
| Problem | Wrong Answer | What Went Wrong? | Right Answer |
|---|---|---|---|
| 9 + 4 | 12 | Counted 9 as one of the 4 | 13 |
| 16 - 7 | 23 | Added instead of subtracted | 9 |
| 7 + 8 | 51 | Wrote 15 backward | 15 |
| 13 - 5 | 9 | Counted 13 when counting back | 8 |
✏️ Practice
Find the correct answer for each problem. Watch out for common mistakes!
Problem 1
Need a hint?
Start AT 9, then count 5 more: 10, 11, 12, 13, 14
Problem 2
Need a hint?
Remember: subtract means the answer is SMALLER than 15
Problem 3
Need a hint?
Make sure you write the teen number correctly - the 1 comes FIRST!
Problem 4
Need a hint?
Count back from 14 (but don't count 14!): 13, 12, 11, 10, 9, 8, 7, 6
Problem 5
Need a hint?
Start at 7, count 5 more: 8, 9, 10, 11, 12. Check: Does 12 - 5 = 7? Yes!
Problem 6
Need a hint?
20 take away 9. Count back 9 from 20 (don't count 20!)
Problem 7
Need a hint?
Start at 9, count 7 more. Write carefully - 16, not 61!
Problem 8
Need a hint?
Check your answer: 5 + 6 should equal 11!
✅ Check Your Understanding
Can you identify these mistakes?
Self-Check Questions
- When you add, should your answer be bigger or smaller than the numbers you started with?
- When you subtract, should your answer be bigger or smaller than the first number?
- What's the best way to check an addition problem?
- How do you write the number "fifteen" - 15 or 51?
Check Your Answers
- Add = answer is BIGGER
- Subtract = answer is SMALLER
- Check addition by subtracting
- Fifteen = 15 (1 comes first for teen numbers)
🚀 Next Steps
- Always double-check your work using the opposite operation
- Take your time - rushing causes mistakes!
- When you're ready, take the Unit Quiz to test your skills