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1st Grade · 1.NBT.C.6

Subtracting tens in 1st grade

Subtracting tens means taking a number like 20 or 50 away from a number like 40 or 80. In 1st grade, your child learns to see each number as a count of tens. Then 80−5080 - 50 becomes 8 tens take away 5 tens, which leaves 3 tens, or 30.

What the standard says

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

1.NBT.C.6 Subtract 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.

What it means, in plain words

Subtracting tens in 1st grade uses the multiples of 10 from 10 to 90. A multiple of 10 is a number made only of tens, with 0 ones, such as 20 or 60. The answer can be 0, as in 40−4040 - 40, but it is never less than 0.

Children start with a picture: rods of ten blocks, or bundles of ten straws. They cross out the tens they take away and count the tens that are left. Then they write the matching equation, such as 70−20=5070 - 20 = 50. They also say why it works: 7 tens minus 2 tens is 5 tens.

A second way uses addition. To find 90−4090 - 40, ask how much to add to 40 to reach 90. Counting up by tens from 40 goes 50, 60, 70, 80, 90, which is 5 tens, so the answer is 50.

Your child should be able to:

  • Subtract a multiple of 10 from another, such as 80−3080 - 30
  • Show the subtraction with ten-rods, bundles or a drawing
  • Explain the answer as a number of tens
  • Count up by tens to solve or check a subtraction

Example

What is 70−4070 - 40?

  1. 70 is 7 tens and 40 is 4 tens.
  2. 7 tens take away 4 tens leaves 3 tens.
  3. 3 tens is 30.
  4. 70−40=3070 - 40 = 30, so the answer is 30.

Example

A shelf holds 80 books. A class borrows 20 of them. How many books are left on the shelf?

  1. 80 is 8 tens and 20 is 2 tens.
  2. 8 tens take away 2 tens leaves 6 tens.
  3. 6 tens is 60.
  4. Check by adding: 20+60=8020 + 60 = 80.
  5. There are 60 books left on the shelf.

Where children usually get stuck

  • For 50−2050 - 20, your child writes 3.

    The child subtracts the digits, 5 minus 2, and forgets that they count tens. The answer is 3 tens, which is 30.

    What helps. Ask the question in tens out loud: 5 tens take away 2 tens is how many tens? Then ask what number 3 tens makes.

  • To find 70−3070 - 30, your child counts back by ones from 70 and lands on 41 or 39.

    Thirty single steps back is a long count, and one slip changes the answer. The child is treating 30 as thirty ones instead of 3 tens.

    What helps. Lay out 7 dimes or 7 bundles of ten straws. Take 3 away and count what is left by tens: 10, 20, 30, 40.

  • In a story where 60 birds sit on a wire and 20 fly away, your child adds and writes 80.

    The child takes the two numbers from the story and adds them, without picturing what happens to the birds.

    What helps. Act the story out with 6 bundles or 6 dimes. Before any numbers get written, ask whether there will be more or fewer at the end.

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.

  1. 1Warm-up
    Look at the subtraction equations below. Select every equation that is correct.
    A
    40−10=\displaystyle 40 - 10 = {}
    30\displaystyle 30
    B
    30−20=\displaystyle 30 - 20 = {}
    50\displaystyle 50
    C
    60−20=\displaystyle 60 - 20 = {}
    40\displaystyle 40
    Show the answer and how to solve it

    The correct equations are 40−10=3040 - 10 = 30 and 60−20=4060 - 20 = 40. The equation 30−20=5030 - 20 = 50 is wrong because 33 tens take away 22 tens leaves 11 ten, which is 1010.

    1. 1Check the first equation. 44 tens take away 11 ten leaves 33 tens, so the answer is 3030. This equation is CORRECT.
    2. 2Check the second equation. 33 tens take away 22 tens leaves 11 ten, so the answer should be 1010, not 5050. This equation is WRONG.
    3. 3Check the third equation. 66 tens take away 22 tens leaves 44 tens, so the answer is 4040. This equation is CORRECT.
    4. 4Select the two correct equations and leave the wrong one unselected.

    Common mistake. A common mistake is to add instead of subtract. For 30−2030 - 20, the equation should subtract, so the answer is 1010, not 5050.

    The idea underneath. When you subtract tens from tens, you can use the basic subtraction fact and add a zero. For example, 4−1=34 - 1 = 3 tells you that 40−10=3040 - 10 = 30.

  2. 2On level
    Maya wrote a sentence to explain a subtraction with tens. The equation and Maya's sentence are shown below.
    60−30=30\displaystyle 60 - 30 = 30
    Maya's sentence: "I had 6 tens, took away 3 tens, now I have 3 tens."
    Select the statement that best describes Maya's sentence.
    A
    Maya wrote the wrong take-away number.
    B
    Maya wrote the wrong ending number.
    C
    Maya wrote the wrong starting number.
    D
    Maya wrote everything correctly.
    Show the answer and how to solve it

    The answer is 'Maya wrote everything correctly.' The equation 60−30=3060 - 30 = 30 means: 6 tens, take away 3 tens, leaves 3 tens. Maya's starting number (6 tens), take-away number (3 tens), and ending number (3 tens) all match the equation.

    1. 1Look at the equation. It shows 60 minus 30 equals 30. Each number tells how many tens.
    2. 2Each number written in tens:
    3. 3Now read Maya's sentence: "I had 6 tens, took away 3 tens, now I have 3 tens." Compare each part to the equation.
    4. 4The starting number 6 tens matches 60. The take-away number 3 tens matches 30. The ending number 3 tens matches 30. Every part of Maya's sentence matches the equation, so Maya wrote everything correctly.

    Common mistake. A common mistake is picking one of the error choices without checking all three parts. Students may scan for a number that looks out of place instead of comparing each part to the equation. Here, 60 is 6 tens, 30 is 3 tens, and 30 is 3 tens, all three match.

    The idea underneath. An equation like 60−30=3060 - 30 = 30 can be read as a tens story: 6 tens, take away 3 tens, leaves 3 tens. To check a sentence, match each tens count to the matching number in the equation, one by one.

  3. 3On level
    The base-ten blocks below show a subtraction.
    -
    What number is left?
    Show the answer and how to solve it

    Answer. 50

    The answer is 50. There were 6 tens to start, and 1 ten was taken away. 6 take away 1 is 5 tens, and 5 tens is the number 50.

    1. 1Count the cyan tens rods. There are 6 tens. That is the number 60.
    2. 2Then 1 ten is taken away. 6 take away 1 is 5. There are 5 tens left.
    3. 35 tens is the same as the number 50. The answer is 50.

    Common mistake. A common mistake is typing 5 instead of 50. There are 5 tens left, but 5 tens means 5 groups of ten — the number 50, not 5.

    The idea underneath. 1 ten rod is the same as the number 10. When you have 5 tens, the number is 50 — count the tens, then count by tens.

  4. 4Stretch
    Look at this subtraction.
    90−60=30\displaystyle 90 - 60 = 30
    Drag a tile into each blank to finish the sentence.
    Tap or click a tile to drop it into the highlighted blank, or tap an empty blank first to choose a different one. Tap a filled blank to send its tile back.
    I had
    tens, took away
    tens, now I have
    tens.
    30
    9
    6
    3
    60
    90

    Tap an item to place it in the next empty slot, or drag it to a specific slot. With a keyboard: select an item with Enter or Space, then select a slot to place it. Press Delete or Backspace on a slot to remove an item.

    Show the answer and how to solve it

    Drag 99 into the first blank, 66 into the second blank, and 33 into the third blank. The finished sentence is: I had 99 tens, took away 66 tens, now I have 33 tens.

    1. 1Look at the subtraction.
    2. 2Each number is made of tens. 9090 is 99 tens. 6060 is 66 tens. 3030 is 33 tens.
    3. 3So the sentence is: I had 99 tens, took away 66 tens, now I have 33 tens.

    Common mistake. A common mistake is dragging 9090 into the first blank instead of 99. The blank asks for the number of tens, not the whole number. Since 9090 means 99 tens, the digit 99 goes in the blank.

    The idea underneath. Every multiple of ten can be counted as a number of tens. 9090 is the same as 99 tens, and 6060 is the same as 66 tens, so subtracting tens works just like subtracting the small numbers.