Work Out the Missing Number in Additions in Your Head

In this activity, we will be working 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 means 50 + 26 because two negative signs next to each other make a positive.

 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 - don't be put off by the decimal!

1 - 0.81 = 1.00 - 0.81 = 0.19 

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

 

Shall we have a go at some questions now?