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.
Which number goes where the ? is?
Here we say 3... 4... 5... 6... 7, so the 5 is where the ? is.
Don't be put off by the fact that several numbers are missing, just count along until you reach the question mark - that's the one you need.
Let's use your detecting skills and get started on the questions.