Chapter 11 - Probability and Statistics - 11-2 Probability - Practice and Problem-Solving Exercises - Page 686: 23

$39\%$

For mutually exclusive events, $P(S\text{ and T}) = 0$, so we have: $$\begin{aligned}\ P(S\text{ or }T)& = P(S) + P(T) - P(S\text{ and T}) = P(S) + P(T)\\ P(S\text{ or }T)& = 12\% + 27\%\\ & = 0.12 + 0.27 \\ &= 0.39\\ &=39\%.\end{aligned}$$