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1st Grade · 1.OA.D.8

Finding the missing addend in 1st grade

A missing addend is a number left out of an addition, like the blank in 6+?=106 + ? = 10. An addend is one of the numbers being added. In 1st grade, your child finds the unknown number in an addition or subtraction equation, wherever the blank sits.

What the standard says

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

1.OA.D.8 Determine the unknown whole number in an addition or subtraction equation relating three whole numbers.

What it means, in plain words

Missing number addition problems put the blank in different places: 7+?=127 + ? = 12, ?+4=9? + 4 = 9 or 10=3+?10 = 3 + ?. Subtraction has them too, such as 15−?=1015 - ? = 10 or ?−5=4? - 5 = 4. Each equation links three numbers: two parts and a whole. One of the three is hidden.

The first step is to find the whole, the total of the two parts. In an addition, it is the number on its own on one side of the equal sign. In a subtraction, it is the starting number. To find a missing addend, your child counts up from the part shown to the whole, or uses a subtraction fact. For 7+?=127 + ? = 12, 7 and 5 make 12, so the missing addend is 5.

Your child can check any answer by putting it in the blank. If 7+57 + 5 makes 12, the equation is true. This check ties missing addend work to the meaning of the equal sign: both sides have the same amount.

Your child should be able to:

  • Find the missing number in an addition, wherever the blank is
  • Find the missing number in a subtraction, such as 15−?=1015 - ? = 10
  • Tell which number in an equation is the whole and which are the parts
  • Check an answer by putting it in the blank and seeing if the equation is true

Example

What number makes 6+?=136 + ? = 13 true?

  1. 13 is the whole. 6 is one part.
  2. Count up from 6 to 13: 7, 8, 9, 10, 11, 12, 13. That is 7 counts.
  3. Check: 6+7=136 + 7 = 13. The missing addend is 7.

Example

What number makes ?−4=8? - 4 = 8 true?

  1. The blank is the starting number, so it is the whole.
  2. Taking 4 away leaves 8, so the whole is 4 and 8 together.
  3. 8+4=128 + 4 = 12.
  4. Check: 12−4=812 - 4 = 8. The missing number is 12.

Where children usually get stuck

  • For 7+?=107 + ? = 10, your child writes 17.

    The child sees a plus sign and adds the two numbers shown. The blank was read as "the answer" instead of as a part.

    What helps. Put 10 pennies on the table and cover some with a cup so that 7 still show. Ask how many are under the cup.

  • For ?−3=6? - 3 = 6, your child writes 3.

    The child takes the smaller number from the bigger one, the way most subtractions look. Here the blank is the starting amount, so it has to be bigger than both numbers shown. The answer is 9.

    What helps. Tell it as a story: there were some cookies, 3 got eaten and 6 are left. Ask how many cookies there were at the start.

  • Your child solves 4+?=94 + ? = 9 but gets stuck on 9=4+?9 = 4 + ?.

    The child expects the total on the right of the equal sign. When the whole moves to the left, the problem looks new, even though it is the same problem.

    What helps. Write both forms one under the other. Ask your child to circle the whole in each, then find the missing part the same way.

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
    Mia is filling in the missing number in this subtraction equation.
    8−00‾=3\displaystyle 8 - \underline{\phantom{00}} = 3
    What number is missing?
    Show the answer and how to solve it

    Answer. 5

    The missing number is 55 because 8−5=38 - 5 = 3.

    1. 1Look at the equation. It shows 8 minus some number that equals 3. We need to find the number that was taken away.
    2. 2Think: 'What number do I take away from 8 to leave 3?' One way is to count back from 8 to 3. That is 5 jumps back. Another way is to subtract: 8 minus 3 is 5.
    3. 3The missing number is 5.

    Common mistake. A common mistake is adding 8 and 3 to get 11 because both numbers are visible. Instead, ask: what number plus 3 makes 8? Or count up from 3 to 8 to see how much was taken away.

    The idea underneath. When the number taken away is unknown, you can find it by counting up from the result to the start, or by subtracting the result from the start.

  2. 2On level
    Aisha is finding the missing number.
    12−□=3\displaystyle 12 - \square = 3
    Aisha says the missing number is 3.
    Is Aisha right?
    A
    Yes. The missing number is 3.
    B
    No. The missing number is 12.
    C
    No. The missing number is 15.
    D
    No. The missing number is 9.
    Show the answer and how to solve it

    Aisha is not right. The missing number is 99, because 12−9=312 - 9 = 3. Aisha picked 33, but 33 is what is left after taking away, not the number that was taken away.

    1. 1Aisha is looking for the missing number in 12−□=312 - \square = 3. The missing number is what we take away from 1212 to leave 33.
    2. 2Try 99. Take away 99 from 1212 and you have 33 left. So the missing number is 99.
    3. 3Aisha said the missing number is 33. But 33 is what is left, not what was taken away. So Aisha is not right. The correct answer is: No. The missing number is 99.

    Common mistake. A common mistake is thinking the missing number is the number on the right of the equal sign. Aisha saw 33 and said the missing number is 33. But 33 is what is left after taking away. The number that was taken away is 99, since 12−9=312 - 9 = 3.

    The idea underneath. When an equation looks like 12−□=312 - \square = 3, the missing number is what gets taken away. To find it, subtract: 12−3=912 - 3 = 9. The number on the right of the equal sign is what is LEFT, not the missing number.

  3. 3On level
    Solve the addition equation. Type the number that completes the equation.
    4+5\displaystyle 4 + 5
    =\displaystyle =
    Show the answer and how to solve it

    The missing total is 99, because 4+5=94 + 5 = 9.

    1. 1Look at the addition equation. The blank goes after the equal sign, so the blank is the total of 44 and 55.
    2. 2Add 44 and 55 by counting on from 55: 6,7,8,96, 7, 8, 9.
    3. 3The number that completes the equation is 99.

    Common mistake. A common mistake is typing one of the addends (like 44 or 55) instead of the total. The blank is the sum — the total when you put the two addends together.

    The idea underneath. The equal sign means "the same as." The number in the blank must make both sides of the equation have the same value.

  4. 4Stretch
    4+00‾=11\displaystyle 4 + \underline{\phantom{00}} = 11
    What number goes in the blank?
    Show the answer and how to solve it

    Answer. 7

    The missing number is 77 because 4+7=114 + 7 = 11.

    1. 1Look at the equation. The number 4 and a missing number add up to 11.
    2. 2Start at 4 and count up until you reach 11.
    3. 3Counting up from 4 to 11 takes 7 steps, so the missing number is 7.

    Common mistake. A common mistake is writing 11 because that is the total shown in the equation, or writing 4 because that is the number already given. The answer is the missing number that adds with 4 to make 11.

    The idea underneath. Finding a missing addend means asking how many more are needed to reach the total. You can count up from the given number, or think of the related subtraction.