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Step-by-step practice: Maths · Level 1Chapter 8 · Adding and taking away within 10
Step-by-step Maths Level 1 · Adding and taking away within 10 · Lesson 8.3

Step-by-step Maths Level 1: Finding the missing number

Part of the Step-by-step Maths Level 1 adding and taking away within 10 chapter. Chapter overview · whole chapter on one page

8.3

Finding the missing number

Level 1
Find a missing number in a sum

Some sums have a box. The box is a missing number.

For a sum with a box, ask: “What do I need to make it right?”

Key ideas

  • A box is a missing number.
  • For + sums, count on from the known part to the total.
  • For − sums, add back the number taken away.
Worked example 8.3.1
Fill in the box: 4 + □ = 9

Method. Find the missing part. Ask what goes with the known part to make the whole.

  1. Look at the whole and the part you know. The whole is 9 and the known part is 4.
    9 and 4
  2. Count on from the known part to the whole. Or take the known part away from the whole.
    9 - 4 = 5
  3. Write the missing part.
    5

Check. Put the two parts back together: 4 and 5 make 9.

Watch out. The two parts always add up to the whole.

Worked example 8.3.2
Fill in the box: 10 − □ = 6

Method. Ask: what take away gives 6? Add back to find the start.

  1. Read the sum. Ten take away the box is 6.
    10 - ? = 6
  2. Add back the answer to the start. Count on from 6 to 10.
    6 + 4 = 10
  3. Write the missing number.
    4

Check. 10 - 4 = 6. It works.

Watch out. The box is the number taken away.

Worked example 8.3.3
Fill in the box: □ − 3 = 4

Method. The box is the start number. Add back the 3.

  1. Read the sum. The box take away 3 is 4.
    ? - 3 = 4
  2. Add back the 3. Count on 3 from 4 to find the start.
    4 + 3 = 7
  3. Write the missing number.
    7

Check. 7 - 3 = 4. It works.

Watch out. Add back to find the start number.

Watch out

Watch out. Do not just write the number you see. Ask what the box must be to make the sum right.

Exam tip

Grown-up tip. Cover one number in a sum with your hand. Ask: “What number was hidden?” Check by reading the whole sum again.

Exercise 8C  Finding the missing number

Non-calculator
1
FFill in the box: 5 + □ = 6
2
FFill in the box: □ + 1 = 4
3
FFill in the box: 9 − □ = 6
4
FFill in the box: □ − 3 = 1
5
FFill in the box: 6 + □ = 7
6
FFill in the box: □ + 6 = 9
7
FFill in the box: 10 − □ = 8
8
FFill in the box: □ − 3 = 7
9
FFill in the box: 1 + □ = 8
10
FFill in the box: □ + 2 = 10
11
FFill in the box: 6 − □ = 2
12
FFill in the box: □ − 3 = 2
13
FFill in the box: 2 + □ = 3
14
FFill in the box: □ + 1 = 5
15
PSFill in the box: 6 − □ = 4
16
RFill in the box: □ − 7 = 3

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