Answer
$.8333$
Work Step by Step
We are interested in the event $(E)$ that a successful author is established.
The sample size is $N=30$, (There are 30 successful authors)
The frequency of $E$ is $fr(E)=25$ (There are 25 successful established authors)
The relative frequency of $E$ is $P(E)=\frac{fr(E)}{N}=\frac{25}{30}\approx .8333$