Early Math · Grade 1

Adding Tens and Ones Within 100

Quick answer

Place value makes adding within 100 manageable. Ten more than a number adds one to its tens digit, and ten less takes one away, with no counting. Tens add to tens and ones add to ones: 46 + 30 is 7 tens and 6 ones, or 76. When the ones add up to 10 or more, ten of them make a new ten: 27 + 6 has 13 ones, which is 1 ten and 3 ones, so the sum is 33.

What you'll learn

  • Find 10 more and 10 less than a two-digit number without counting
  • Add and subtract multiples of 10
  • Add a one-digit number or a multiple of 10 to a two-digit number, making a ten when needed

Ten more and ten less

Here is 3434: 33 tens and 44 ones.

Thirty-four, then ten more On top, three rods of ten and four single cubes. Below, four rods of ten and the same four cubes.
Thirty-four, then ten more

Add one more ten. Now there are 44 tens and still 44 ones. That is 4444.

34+10=4434 + 10 = 44

Ten more adds 11 to the tens digit. Ten less takes 11 away from it: 34−10=2434 - 10 = 24. The ones stay the same, so you do not need to count.

Add and subtract tens

Tens work like ones. 33 tens and 44 tens make 77 tens.

30+40=7030 + 40 = 70

Taking away tens works the same way. 88 tens take away 33 tens leaves 55 tens: 80−30=5080 - 30 = 50.

Why tens go with tens

A ten and a one are not the same size. So 33 tens and 44 ones are not "77" of anything. They are 3434.

That is why we add tens to tens and ones to ones. Each kind is counted with its own kind.

Make a new ten

Sometimes the ones make 1010 or more. Look at 27+627 + 6.

Twenty-seven and six Base-ten blocks: two rods of ten and thirteen single cubes, ten in one column and three in the next.
Twenty-seven and six

The ones are 7+6=137 + 6 = 13. That is too many ones to write in one place. But ten of them make a new ten. So 1313 ones is 11 ten and 33 ones.

Now there are 33 tens and 33 ones:

27+6=20+13=3327 + 6 = 20 + 13 = 33

Worked examples

Common mistakes

Practice problems

  1. What is 1010 more than 6262?

    Answer

    7272

    Full solution

    66 tens become 77 tens. The 22 ones stay.

  2. What is 1010 less than 4545?

    Answer

    3535

    Full solution

    44 tens become 33 tens. The 55 ones stay.

  3. Find 50+3050 + 30.

    Answer

    8080

    Full solution

    55 tens and 33 tens make 88 tens, which is 8080.

  4. Find 70−2070 - 20.

    Answer

    5050

    Full solution

    77 tens take away 22 tens leaves 55 tens.

  5. Find 42+642 + 6.

    Answer

    4848

    Full solution

    Add the ones: 2+6=82 + 6 = 8. The tens stay at 44.

  6. Find 25+4025 + 40.

    Answer

    6565

    Full solution

    Add the tens: 2+4=62 + 4 = 6 tens. The 55 ones stay.

  7. Find 49+349 + 3.

    Answer

    5252

    Full solution

    49+1=5049 + 1 = 50, and 22 more is 5252. The ones made a new ten.

  8. Find 60−6060 - 60.

    Answer

    00

    Full solution

    66 tens take away 66 tens leaves no tens and no ones.

  9. Find 36+736 + 7.

    Answer

    4343

    Full solution

    The ones are 6+7=136 + 7 = 13: 11 ten and 33 ones. The tens are 3+1=43 + 1 = 4. So the answer is 4343.

  10. Ben says 54+3=8454 + 3 = 84. What went wrong?

    Hint

    Is the 33 tens or ones?

    Answer

    He added the 33 to the tens. It is ones: 54+3=5754 + 3 = 57.

    Full solution

    33 is a one-digit number, so it means 33 ones. Add it to the 44 ones: 4+3=74 + 3 = 7.

    The 55 tens stay, so 54+3=5754 + 3 = 57.

Frequently asked questions

How do I find 10 more than a number?

Add one to the tens digit and keep the ones. 10 more than 34 is 44.

What is 30 + 40?

3 tens and 4 tens make 7 tens, which is 70.

Why do tens add to tens and ones to ones?

Tens and ones are different sizes, so they are counted apart. 3 tens and 4 ones are not 7 of anything; they are 34.

What happens when the ones add up to more than 9?

Ten of the ones make a new ten. 27 + 6 has 7 + 6 = 13 ones, which is 1 ten and 3 ones, so the answer is 33.

What is 90 − 40?

9 tens take away 4 tens leaves 5 tens, which is 50.

What to learn next

Standards alignment

This lesson covers the following Common Core State Standards for Mathematics.

  • CCSS.MATH.CONTENT.1.NBT.C.4Number and Operations in Base TenAdd within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used. Understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.
  • CCSS.MATH.CONTENT.1.NBT.C.5Number and Operations in Base TenGiven a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
  • CCSS.MATH.CONTENT.1.NBT.C.6Number and Operations in Base TenSubtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90 (positive or zero differences), using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.
For teachersTick to print a blank worksheet; leave it clear to print the lesson with the full answer key.