Answer
Fill the blank with:
... $P(E)=\displaystyle \frac{a}{a+b}$ ...
Work Step by Step
If odds in favor of $E$ are $a$ to $b$, then $P(E)=\displaystyle \frac{a}{a+b}$.
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Fill the blank with:
... $P(E)=\displaystyle \frac{a}{a+b}$ ...