Answer
$P(placebo/flushing)=\frac{7}{725}\approx0.0097$.
It is an unusual event because $P(placebo/flushing)\approx0.0097\lt0.05$
Work Step by Step
Total of subjects that received a placebo: 725. So, $N(S)=725$.
Event: "subject receiving a placebo experiencing flushing". $N(E)=7$.
Using the Classical Method:
$P(E)=P(placebo/flushing)=\frac{N(E)}{N(S)}=\frac{7}{725}\approx0.0097$
