Let A be the event S happens on the first try, note [tex]A^c[/tex] is the complement of A.

Let B be the event S happens on the second try.

I reasoned it to be a 79 percent chance of S happening.

1. A and B are mutually exclusive, in other words if A happens B does not, vise versa.

2. The probability of event B happening depends on event A not happening.

Thus [tex]P(B \cap A^c) = .39[/tex], so P(S) = P(A) + [tex]P(B \cap A^c)[/tex] = .79.

So I reason a 79 percent chance of event S.

Thanks