From any addition fact such as
1300 + 5200 = 6500
we can work out two subtraction facts.
Here we get
6500  1300 = 5200
and
6500  5200 = 1300
This is handy because we can use additions to check subtractions.
Example 1
Ahmed says that
2800  1100 = 

Use addition to check if he is right or wrong.
Answer
1700 + 1100 = 2800
So Ahmed is right.
Example 2
Bill says that
6500  3700 = 

Use addition to check if he is right or wrong.
Answer
3200 + 3700 = 6900 not 6500
So Bill is wrong.