Answer: [tex]\frac{4}{7}[/tex]
Step-by-step explanation:
P(a) or P(b) = P(a) + P(b)
This means that the probability of a or b is the same of the probability of a plus b
So let's see the probability of a, selecting a professor
[tex]\frac{12}{12+6+9+8} \\\\\frac{12}{35}[/tex]
We won't simplify just yet so that addition will be easier later
Now let's see the probability of selecting b, an instructor
[tex]\frac{8}{12+6+9+8} \\\\\frac{8}{35}[/tex]
Now add the 2 probabilities together
[tex]\frac{12}{35} +\frac{8}{35} =\frac{20}{35}[/tex]
Simplify.
[tex]\frac{20}{35} =\frac{4}{7}[/tex]