Question 6
An information station employs four different codes, $A$, $B$, $C$ and $D$, for communication, but each week uses only one of them. The code used in a definite week is randomly selected with equal chance among the three ones that have not been used in the last week. Suppose the code used in the first week is $A$. Then the probability that $A$ is also used in the seventh week is _______. (expressed as an irreducible fraction)