To find how many people at the convention are bored by cable news, we start by noting the fraction of people who are bored, which is given as \( \frac{5}{8} \).
Next, we will apply this fraction to the total number of people at the convention, which is \( 120 \).
We can calculate the number of bored people by multiplying the total number of people by the fraction of people who are bored:
\[ \text{Number of bored people} = \frac{5}{8} \times 120 \]
To perform the multiplication, we first calculate \( 120 \div 8 \):
\[ 120 \div 8 = 15 \]
Now, we multiply \( 15 \) by \( 5 \):
\[ 15 \times 5 = 75 \]
Thus, the number of people at the convention who are bored by cable news is
\[ \boxed{75} \]