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Step-by-step practice: Maths · Level 2Chapter 6 · Taking away within 20
Step-by-step Maths Level 2 · Taking away within 20 · Lesson 6.5

Step-by-step Maths Level 2: Missing numbers and checking

Part of the Step-by-step Maths Level 2 taking away within 20 chapter. Chapter overview · whole chapter on one page

6.5

Missing numbers and checking

Level 2
Find a missing number in a subtraction, Use addition to check a subtraction

A subtraction can have a missing number. Ask: “What do I take away to get from the start to the answer?”

To check any answer, add the number you took away back on to the answer.

Key ideas

  • Subtraction and addition undo each other.
  • Check by adding back.
  • A missing number is found by working backwards.
Worked example 6.5.1
Write the missing number: 15 - ? = 9

Method. Find the number to take away. Check by adding it back.

  1. Start with 15. It is the number we begin with.
    15
  2. Find how many are taken away to reach 9. Count back, or take the answer away from the start.
    15 - 9 = 6
  3. Write the missing number.
    6

Check. Add back: 9 + 6 = 15. It works.

Watch out. The missing number is the one that gets you from the start to the answer.

Worked example 6.5.2
16 - 9 = ? Check your answer by adding.

Method. Work out the subtraction. Then add the number you took away to the answer.

  1. Work out 16 - 9. Count back from the first number.
    16 - 9 = 7
  2. Check by adding. If the answer is right, adding back gives the first number.
    7 + 9 = 16
  3. Write the answer.
    7

Check. 7 + 9 = 16, so the answer is right.

Watch out. Subtraction and addition undo each other. Use addition to check.

Worked example 6.5.3
Write the missing number: 13 - ? = 9

Method. Find the number to take away. Check by adding it back.

  1. Start with 13. It is the number we begin with.
    13
  2. Find how many are taken away to reach 9. Count back, or take the answer away from the start.
    13 - 9 = 4
  3. Write the missing number.
    4

Check. Add back: 9 + 4 = 13. It works.

Watch out. The missing number is the one that gets you from the start to the answer.

Watch out

Watch out. In 15 - ? = 9, the missing number is 6, not 9. The answer is the number after the equals sign.

Exam tip

Grown-up tip. Use addition to check every subtraction. Ask: “Does the answer add back to the start?”

Exercise 6E  Practice: missing numbers and checking

Non-calculator
1
F17 - 10 = ? Check your answer by adding.
2
F12 - 4 = ? Check your answer by adding.
3
FWrite the missing number: 7 - ? = 3
4
FWrite the missing number: 10 - ? = 7
5
F12 - 5 = ? Check your answer by adding.
6
FWrite the missing number: 19 - ? = 15
7
F17 - 8 = ? Check your answer by adding.
8
F11 - 1 = ? Check your answer by adding.
9
F15 - 10 = ? Check your answer by adding.
10
FWrite the missing number: 16 - ? = 15
11
FWrite the missing number: 13 - ? = 2
12
FWrite the missing number: 18 - ? = 8
13
FWrite the missing number: 7 - ? = 6
14
FWrite the missing number: 9 - ? = 4
15
F18 - 3 = ? Check your answer by adding.
16
FWrite the missing number: 10 - ? = 1

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