Mathematics — Additions

Working Out Missing Numbers Involving Additions

Age: 7 (Kenyan Primary) — Learning goal: Find the missing number in addition sentences like 4 + ? = 9.

What is a "missing number"?

A missing number is a blank in a number sentence. We work out the blank so the sentence is correct. Example: 6 + ? = 10. What number goes in the ? to make 10?

Three easy ways to find the missing number

1. Use number bonds: Think of parts that make a whole. If whole is 10 and one part is 6, the other part is 4.
2. Count on: Start from the known number and count forward until you reach the total.
3. Use subtraction (inverse): If 6 + ? = 10, do 10 − 6 = 4. The answer is 4.

Visual way — Counters

We can draw counters (dots) to see how many are missing.

Another example — Using subtraction

Example 2: 8 + ? = 11

We can use subtraction: 11 − 8 = 3. So ? = 3.

Steps to solve a missing number

• Read the number sentence: which number is missing?
• If you know the total, take away (subtract) the known part to find the missing part.
• Or draw counters and count how many are left.
• Write the missing number and check: does the addition make the total?

Practice — Try these

Fill in the missing number:

1. 5 + ? = 9
2. ? + 4 = 10
3. 7 + ? = 12
4. ? + 3 = 8
5. 6 + ? = 6
6. ? + 5 = 13
7. 9 + ? = 11
8. ? + 7 = 14
9. 2 + ? = 7
10. ? + 9 = 15

Answers (check your work)

1. 5 + 4 = 9
2. 6 + 4 = 10
3. 7 + 5 = 12
4. 5 + 3 = 8
5. 6 + 0 = 6
6. 8 + 5 = 13
7. 9 + 2 = 11
8. 7 + 7 = 14
9. 2 + 5 = 7
10. 6 + 9 = 15

Teacher / Parent tips

• Use real objects (stones, beads, mangoes) for children to move and count.
• Ask the child to explain in simple words how they found the missing number.
• Start with small totals (≤ 20) and practise number bonds to 10 and 20.

Well done! Practise a little every day and you'll get faster at finding missing numbers.