Doubling means: add the number plus itself.
The filled in number shows the answer.
Example
4+4=8
For parents
What does this exercise practice?
Doubling trains adding equal addends and lays the foundation for the times table with the 2.
Common pitfalls
Children sometimes forget that doubling is exactly the same as 'plus itself'. Writing the same number down twice helps.
How to practice at home
When setting the table: if each person gets two forks, how many forks is that altogether? Doubling becomes very concrete this way.