1st Grade · 2nd Grade · 1.NBT.C.5 · 2.NBT.B.8

10 more, 10 less in 1st and 2nd grade

10 more, 10 less is a mental math skill: finding the number that is ten bigger or ten smaller without counting and without paper. Mental math means working a problem out in your head. In 1st grade, your child finds 10 more and 10 less than any two-digit number. In 2nd grade, your child adds and takes away 10 or 100 from numbers between 100 and 900.

What the standards say

The official Common Core wording, quoted exactly. The plain version follows.

1.NBT.C.5 Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.

2.NBT.B.8 Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.

What it means, in plain words

10 more, 10 less in 1st grade: only the tens digit changes 1.NBT.C.5

Finding 10 more and 10 less than a two-digit number takes one step. Adding 10 adds one ten, so the tens digit goes up by 1 and the ones digit stays the same. 10 more than 56 is 66. Taking away 10 takes away one ten, so the tens digit goes down by 1, and 10 less than 56 is 46.

The goal is to do this without counting by ones. A hundred chart, a grid of the numbers from 1 to 100 with ten in each row, shows why it works. The number just below any number is 10 more, and the number just above it is 10 less. Teachers often call this ten more, ten less. Children explain it in their own words: one more ten, or one less ten.

Your child should be able to:

Say 10 more than any two-digit number without counting
Say 10 less than any two-digit number without counting
Explain that only the tens digit changes
Find 10 more and 10 less on a hundred chart by moving down or up one row

Example

What is 10 more than 73, and what is 10 less than 73?

73 is 7 tens and 3 ones.
10 more adds one ten: 8 tens and 3 ones is 83.
10 less takes away one ten: 6 tens and 3 ones is 63.
10 more than 73 is 83, and 10 less than 73 is 63.

10 more, 10 less in 2nd grade: now with 100 more and 100 less 2.NBT.B.8

In 2nd grade the numbers grow to three digits, from 100 to 900, and children add or take away 100 as well as 10. The full skill becomes 10 more, 10 less, 100 more and 100 less. Adding 10 changes the tens digit. Adding 100 changes the hundreds digit, the first digit of a three-digit number.

The other digits stay the same, so 100 more than 438 is 538, and 10 less than 438 is 428. The tricky numbers are the ones where a change crosses into a new hundred. 10 more than 495 is 505, because 9 tens and 1 more ten make 10 tens, and 10 tens is one more hundred.

Your child should be able to:

Add 10 or 100 to a number from 100 to 900 without paper
Take away 10 or 100 from a number from 100 to 900 without paper
Say which digit changes when adding 10 and which changes when adding 100
Handle numbers with a 9 or a 0 in the tens place, such as 10 more than 495

Example

What is 100 less than 642, and what is 10 more than 642?

642 is 6 hundreds, 4 tens and 2 ones.
100 less takes away one hundred: 5 hundreds, 4 tens and 2 ones is 542.
10 more adds one ten: 6 hundreds, 5 tens and 2 ones is 652.
100 less than 642 is 542, and 10 more than 642 is 652.

Example

What is 10 less than 504?

504 has 0 tens, so there is no ten to take away yet.
Break one hundred into 10 tens: 504 is now 4 hundreds, 10 tens and 4 ones.
Take away one ten: 4 hundreds, 9 tens and 4 ones is left.
10 less than 504 is 494.

Where children usually get stuck

Your child finds 10 more than 58 by counting up on fingers from 59 and often lands on 67 or 69.
Ten single steps are slow, and one missed or extra step changes the answer. The child has not yet seen that adding ten only changes the tens digit.
What helps. Show 58 cents with 5 dimes and 8 pennies. Add one dime, then read the new amount together: 68 cents.
Your child says that 10 more than 63 is 64.
The child mixes up 10 more and 1 more and changes the ones digit, because that is the digit that changes when counting.
What helps. On a hundred chart, show that 1 more is one step to the right and 10 more is one step down. Ask your child to point first, then say the number.
Your child says that 10 more than 293 is 2103.
The 9 tens and 1 more ten make 10 tens, and the child writes the 10 in the tens place. 10 tens is one hundred, so the hundreds digit goes up and the answer is 303.
What helps. Write the number in three columns marked hundreds, tens and ones. When a column reaches 10, trade the 10 for 1 in the column to its left.

Try four practice questions

Real 1st grade questions from our question bank, easiest first. Answer them right here, check each one, and see a worked solution for any question.

1Warm-up
Lin found 10 more than 34. Lin wrote 44 as the answer.
Lin's work:
$\displaystyle 34 + 10 = 44$
Choose the sentence that best describes Lin's work.
A
Lin's 44 comes from subtracting 1 from the tens digit. The correct answer is 44.
B
Lin's 44 is correct.
C
Lin's 44 comes from adding 1 to the ones digit. The correct answer is 44.
D
Lin's 44 comes from adding 1 to both digits. The correct answer is 44.
Show the answer and how to solve it
Lin's work is correct. Adding 10 to 34 gives 44, and that is exactly what Lin wrote.
1. 1Lin was finding 10 more than 34. She wrote 44 as sher answer. Let's check Lin's work.
2. 2Find the correct answer. When you add 10 to a two-digit number, the tens digit goes up by 1 and the ones digit stays the same. The tens digit of 34 is 3, so it becomes 4. The ones digit (4) stays the same. The correct answer is 44.
3. 3Lin wrote 44, and the correct answer is also 44. The tens digit went from 3 to 4 (up by 1), and the ones digit (4) stayed the same. Lin's answer matches the correct answer exactly.
4. 4The sentence that describes Lin's work is: 'Lin's 44 is correct.'
Common mistake. A common mistake is assuming the work must contain an error. Check each digit: the tens digit went from 3 to 4 (up by 1) and the ones digit stayed at 4, giving 44. Lin's answer is right.
The idea underneath. Adding 10 to a two-digit number changes only the tens digit by 1. The ones digit stays the same.
2On level
What is 10 less than 263?
Show the answer and how to solve it
Answer. 253
10 less than 263 is 253. Only the tens digit changes from 6 to 5; the 2 in the hundreds place and the 3 in the ones place stay the same.
1. 1You need to find 10 less than 263. Subtracting 10 only changes the tens place because 10 is one ten.
2. 2Look at the tens digit in 263. The tens digit is 6. Take away one ten, so the tens digit becomes 5. The hundreds digit (2) and the ones digit (3) stay the same.
3. 3Put the digits together to get the answer:
Common mistake. A common mistake is changing the wrong digit. Some students change the ones digit and write 262, or change the hundreds digit and write 163. To take away 10, change only the tens digit — it goes down by 1.
The idea underneath. When you subtract 10 from a three-digit number, only the tens digit changes — it goes down by 1. The hundreds digit and the ones digit stay the same. This works because 10 is exactly one ten.
3On level
What is 10 more than 71?
A
81
B
72
C
61
D
82
Show the answer and how to solve it
10 more than 71 is 81. The tens digit goes up by 1 (from 7 to 8), and the ones digit stays the same.
1. 1We want to find 10 more than 71. Start with 71.
2. 2When you add 10 to a two-digit number, the tens digit goes up by 1 and the ones digit stays the same. The tens digit of 71 is 7, so it becomes 8. The ones digit 1 stays the same.
3. 3So 10 more than 71 is 81.
Common mistake. A common mistake is adding 1 instead of 10, giving 72. To find 10 more, the tens digit goes up by 1, not the ones digit.
The idea underneath. To add 10 to a two-digit number, just increase the tens digit by 1. The ones digit doesn't change.
4Stretch
Liam wants to find $10$ less than $672$ . He wrote: $672 - 10 = 572$ . Select the statement that best describes Liam's work.
A
Liam's $572$ comes from subtracting $100$ instead of $10$ .
B
Liam's $572$ comes from subtracting $1$ instead of $10$ .
C
Liam's $572$ comes from adding $10$ instead of subtracting.
D
Liam's $572$ is correct.
Show the answer and how to solve it
The correct description is that Liam's $572$ comes from subtracting $100$ instead of $10$ . The right answer to " $10$ less than $672$ " is $662$ .
1. 1Look at what Liam wrote.
2. 2Subtracting $10$ changes only the tens digit. The tens digit of $672$ is $7$ , so subtracting $1$ ten gives $6$ tens. The correct answer is $662$ .
3. 3But Liam wrote $572$ . That is $100$ less than $672$ , not $10$ less. So Liam subtracted $100$ instead of $10$ .
4. 4The statement that describes Liam's work is: Liam's $572$ comes from subtracting $100$ instead of $10$ .
Common mistake. A common mistake is changing the hundreds digit instead of the tens digit. Subtracting $10$ from $672$ changes only the tens digit, giving $662$ .
The idea underneath. When you subtract $10$ , only the tens digit changes — the ones and hundreds digits stay the same.