Answer
$\frac{20}{55}$
Work Step by Step
Events of each horse winning the race are mutually exclusive of each other,
i.e
$P(Scorpion) +P(Starry Avenger) + P(Australian Doll) +
P(Dusty Stake) + P(Outandout) = 0.10 + 0.25 + 0.15 + 0.30 + 0.20 = 1$
Given that Horse Australian Doll and Dusty Stake are scratched from the race
Probability that Outandout will win is:
$\frac{P(Outandout)}{P(Outandout) + P(Scorpion) +P(Starry Avenger)}$
= $\frac{0.20}{0.20+0.10+0.25} = \frac{20}{55}$
