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Work Out the Missing Number in Additions in Your Head

In this worksheet, students must work out the missing number in addition sums.

'Work Out the Missing Number in Additions in Your Head' worksheet

Key stage:  KS 2

Curriculum topic:   Number: Addition and Subtraction

Curriculum subtopic:   Add/Subtract Mentally

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

In this worksheet, you must work out the missing numbers.

 

Example 1

In your head, find the missing number.

26 +
 
= 50

 

Answer

Here it is easy to work out the inverse.

50 - 26 = 24

 

 

Example 2

In your head, find the missing number.

-26 +
 
= 50

 

Answer

Here we use the inverse again, but be careful about the negative sign.  

50 - -26 = 50 + 26 = 76

 

 

Example 3

In your head, find the missing number.

0.81 +
 
= 1

 

Answer

Here it is easy to work out the inverse.

1 - 0.81 = 1.00 - 0.81 = 0.19

It may help to think of 100 - 81 = 19.

Work in your head to find the missing number.

 

20 +
 
= 55

Work in your head to find the missing number.

 

0.2 +
 
= 1

Work in your head to find the missing number.

 

31 +
 
= 100

Work in your head to find the missing number.

 

0.6 +
 
= 1

Work in your head to find the missing number.

 

-27 +
 
= 100

Work in your head to find the missing number.

 

0.26 +
 
= 1

Work in your head to find the missing number.

 

59 +
 
= 100

Work in your head to find the missing number.

 

0.3 +
 
= 1

Work in your head to find the missing number.

 

0.67 +
 
= 1

Work in your head to find the missing number.

 

-20 +
 
= 50
  • Question 1

Work in your head to find the missing number.

 

20 +
 
= 55
CORRECT ANSWER
35
EDDIE SAYS
55 - 20 = 35
  • Question 2

Work in your head to find the missing number.

 

0.2 +
 
= 1
CORRECT ANSWER
0.8
EDDIE SAYS
1 - 0.2 = 0.8
  • Question 3

Work in your head to find the missing number.

 

31 +
 
= 100
CORRECT ANSWER
69
EDDIE SAYS
100 - 31 = 69
  • Question 4

Work in your head to find the missing number.

 

0.6 +
 
= 1
CORRECT ANSWER
0.4
EDDIE SAYS
1 - 0.6 = 0.4
  • Question 5

Work in your head to find the missing number.

 

-27 +
 
= 100
CORRECT ANSWER
127
EDDIE SAYS
100 - -27 = 100 + 27 = 127
  • Question 6

Work in your head to find the missing number.

 

0.26 +
 
= 1
CORRECT ANSWER
0.74
EDDIE SAYS
1 - 0.26 = 0.74
  • Question 7

Work in your head to find the missing number.

 

59 +
 
= 100
CORRECT ANSWER
41
EDDIE SAYS
100 - 59 = 41
  • Question 8

Work in your head to find the missing number.

 

0.3 +
 
= 1
CORRECT ANSWER
0.7
EDDIE SAYS
1 - 0.3 = 0.7
  • Question 9

Work in your head to find the missing number.

 

0.67 +
 
= 1
CORRECT ANSWER
0.33
EDDIE SAYS
1 - 0.67 = 0.33
  • Question 10

Work in your head to find the missing number.

 

-20 +
 
= 50
CORRECT ANSWER
70
EDDIE SAYS
50 - -20 = 50 + 20 = 70
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