Common Mistakes: Place Value to 100
Learn
Everyone makes mistakes when learning! The best learners know what mistakes to watch out for. Let's look at the most common place value mistakes and how to avoid them.
Mistake #1: Mixing Up Tens and Ones
Wrong: "The number 47 has 7 tens and 4 ones"
Right: "The number 47 has 4 tens and 7 ones"
Remember: The FIRST digit (on the left) is tens. The SECOND digit (on the right) is ones.
Mistake #2: Forgetting What a Ten Is Worth
Wrong: "5 tens = 5"
Right: "5 tens = 50"
Remember: Each ten is worth 10, not 1! So 5 tens = 5 x 10 = 50.
Mistake #3: Adding Tens and Ones Wrong
Wrong: "3 tens + 5 ones = 8"
Right: "3 tens + 5 ones = 30 + 5 = 35"
Remember: Convert tens to their value first (3 tens = 30), then add the ones.
Mistake #4: Confusing "10 More" with "1 More"
Wrong: "10 more than 45 is 46"
Right: "10 more than 45 is 55"
Remember: "10 more" changes the tens digit, not the ones digit!
Mistake #5: Comparing Using Only One Digit
Wrong: "38 is greater than 45 because 8 is greater than 5"
Right: "45 is greater than 38 because 4 tens is more than 3 tens"
Remember: Always compare the tens first! Only look at ones when the tens are the same.
Fix the Mistakes
Look at each problem. Find and fix the mistake!
Example 1
Student wrote: The number 62 has 2 tens and 6 ones.
What's wrong? The student mixed up tens and ones.
Correct answer: The number 62 has 6 tens and 2 ones.
Example 2
Student wrote: 10 less than 75 is 74.
What's wrong? The student subtracted 1 instead of 10.
Correct answer: 10 less than 75 is 65.
Example 3
Student wrote: 29 is greater than 31 because 9 is more than 1.
What's wrong? The student only looked at the ones digit.
Correct answer: 31 is greater because 3 tens is more than 2 tens.
Practice: Find and Fix the Errors
Each problem has a mistake. Find it and write the correct answer.
Problem 1
Student answer: The number 58 has 8 tens and 5 ones.
What is the correct answer? _____________________________
Problem 2
Student answer: 4 tens + 3 ones = 7
What is the correct answer? _____________________________
Problem 3
Student answer: 10 more than 67 is 68.
What is the correct answer? _____________________________
Problem 4
Student answer: 7 tens = 7
What is the correct answer? _____________________________
Problem 5
Student answer: 49 is greater than 52 because 9 > 2.
What is the correct answer? _____________________________
Problem 6
Student answer: 10 less than 40 is 39.
What is the correct answer? _____________________________
Problem 7
Student answer: The number with 9 tens and 0 ones is 9.
What is the correct answer? _____________________________
Problem 8
Student answer: 83 and 38 are the same because they have the same digits.
Is this true or false? Explain: _____________________________
Problem 9
Student answer: If I have 56 and add 10, I get 57.
What is the correct answer? _____________________________
Problem 10
Student answer: To find 10 less than 25, I change the ones digit to get 24.
What should the student do instead? _____________________________
Problem 11
Student answer: 6 tens and 12 ones = 612
What is the correct answer? _____________________________
Problem 12
Write your own place value mistake and then fix it!
Mistake: _____________________________
Correct answer: _____________________________
Answer Key
Click to reveal answers
Problem 1: 58 has 5 tens and 8 ones
Problem 2: 4 tens + 3 ones = 40 + 3 = 43
Problem 3: 10 more than 67 is 77
Problem 4: 7 tens = 70
Problem 5: 52 is greater than 49 because 5 tens > 4 tens
Problem 6: 10 less than 40 is 30
Problem 7: 9 tens and 0 ones = 90
Problem 8: False! 83 has 8 tens (80), but 38 only has 3 tens (30). 83 > 38
Problem 9: 56 + 10 = 66
Problem 10: Change the tens digit, not ones. 25 - 10 = 15
Problem 11: 6 tens + 12 ones = 60 + 12 = 72 (or 7 tens and 2 ones)
Problem 12: Answers will vary
Next Steps
- Review any mistakes you made in earlier lessons
- Make a checklist of things to check when solving place value problems
- Take the Unit Quiz to test your knowledge