A concept question about multiples

GRE Quant Problem Solving
1

Let us say we have a set 3,6,9,12,…
We know that, to form every other number, we are adding 3 to its previous number.
We also know that all these numbers are multiples of 3.

But if we have a set like the following.
image
(picture from Greg’s Flashcards)

Which number are the above numbers multiples of? How do we know?

2

You got three prime numbers in the series : 7 , 13 ,19 (which have only 2 multiples : themselves and 1)

3

whoops