Chapter 11 - Counting Methods and Probability Theory - Chapter Summary, Review, and Test - Review Exercises - Page 760: 78

$\frac{3}{4}$

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 $P(E)=\frac{P(E)}{P(\text{not E})+P(E)}=\frac{3P(\text{not E})}{P(\text{not E})+3P(\text{not E})}=\frac{3}{4}$