Addition Input/Output Tables - within 20

Consider the table:

In Out
2 4
7 9
0 2
3 5

Let's find the rule from the table given.

Notice that the given rule makes each number increase.
 

Try using addition.
 

+ 2 = 4

+ 2 = 9

+ 2 = 2

+ 2 = 5
 

Here, 2 is added each time in the input.
 

So, the rule is add 2.