Answer
$\frac{3}{4}$
Work Step by Step
The odds for $E$: $\frac{P(E)}{P(\text{not E})}$. The odds against $E$: $\frac{P(\text{not E})}{P(E)}$
Hence here $P(E)=3P(\text{not E})$. We know that $P(E)=\frac{P(E)}{P(\text{not E})+P(E)}=\frac{3P(\text{not E})}{P(\text{not E})+3P(\text{not E})}=\frac{3}{4}$