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)