Find the missing term in the series below : 6561, 2187, 729, ?, 81, 27

The missing term is 243.

Each term is one-third of the previous : the ratio is 1/3.

These are powers of 3 : 3^8 = 6561, 3^7 = 2187, 3^6 = 729.

The next power 3^5 = 243, then 3^4 = 81 and 3^3 = 27.

So the gap is filled by 243.