This is a case of choosing objects from a set, where order matters.
The number of ways you can choose some objects from a set when order matters is called variations.
The formula of variations is:
Vm,n = m! / (m - n)!
In this case it is the selection of four articles from a set of six articles =>
V6,4 = 6! / (6-4)! = 6*5*4! / 4! = 6*5 = 30
Answer: 30
