Which formula can be used to describe the sequence below?
27,9,3,1,1/3

This is a geometric sequence with an initial term of 27 and a common ratio of 1/3

This just means that each term is 1/3 the term preceding it.

Any geometric sequence can be expressed as:

a(n)=ar^(n-1), a=initial term, r=common ratio, n=term number in this case:

a(n)=27(1/3)^(n-1)