Answer
$\frac{3}{4}$
Work Step by Step
For mutually exclusive events, $P(S\text{ and T}) = 0$, so we have: $$\begin{aligned}
P(S\text{ or }T)& = P(S) + P(T) - P(S\text{ and }T) = P(S) + P(T)\\
P(S\text{ or }T)&= \frac{5}{8} + \frac{1}{8}\\
& = \frac{6}{8}\\
& = \frac{3}{4}.
\end{aligned}$$
