Numbers can be placed in order on a number line like this:
When there are numbers missing, say them in your head as you move along the line.
Example
Match the missing numbers on this number line to the letters.
Answer
Here we say
15... 16... 17... 18... 19... 20... 21... 22... 23... 24... 25
So
A is 20
B is 16
C is 23