Concept:
Probability is defined as:
\[
P = \frac{\text{Favorable outcomes}}{\text{Total outcomes}}
\]
Here, we randomly select one letter from the given word.
Step 1: Count total number of outcomes.
The word TRICKS has letters:
\[
\{T,\ R,\ I,\ C,\ K,\ S\}
\]
Total number of letters:
\[
= 6
\]
Step 2: Count favorable outcomes.
Favorable letters are:
\[
\{T,\ R\}
\]
Number of favorable outcomes:
\[
= 2
\]
Step 3: Calculate probability.
\[
P(\text{T or R}) = \frac{2}{6}
\]
\[
= \frac{1}{3}
\]
Step 4: Final answer.
\[
\boxed{\frac{1}{3}}
\]