Answer :
An arithmetic progression is a progression where the next term is found by multiplying the previous by a constant number called the common ratio, for the given progression:
[tex]4,2,1[/tex]If we use 1/2 as a common ratio we get:
[tex]\begin{gathered} 2=\frac{4}{2} \\ 1=\frac{2}{2} \end{gathered}[/tex]Therefore this is an arithmetic progression and its common ratio is 1/2