Answer
$P(E)=0.4$
Work Step by Step
Event = "Clarice attends the conference" = {Jonh and Clarice, Roberto and Clarice, Clarice and Dominique, Clarice and Marco}. So, $N(E)=4$.
The sample space: S = {John and Roberto, John and Clarice, John and Dominique, John and Marco, Roberto and Clarice, Roberto and Dominique, Roberto and Marco, Clarice and Dominique, Clarice and Marco, Dominique and Marco}. So, $N(S)=10$.
Using the Classical Method:
$P(E)=\frac{N(E)}{N(S)}=\frac{4}{10}=0.4$