To calculate the probability that a person spends their time going to the movies given that they are 31–35 years old, we will use the following formula for conditional probability:
\[ P(A | B) = \frac{P(A \cap B)}{P(B)} \]
In this context:
- \(A\) is the event that a person spends time going to the movies.
- \(B\) is the event that a person is 31–35 years old.
From the table, we find the number of people who are 31–35 years old and go to the movies:
- The number of people aged 31–35 who go to the movies is 21.
The total number of people aged 31–35 is:
\[ 18 \text{ (Bowling)} + 11 \text{ (Dancing)} + 21 \text{ (Movies)} = 50 \]
Now we can calculate the probability that a person spends their time going to the movies given that they are 31–35 years old:
\[ P(\text{Movies} | 31-35) = \frac{\text{Number of people aged 31–35 who go to the movies}}{\text{Total number of people aged 31–35}} = \frac{21}{50} \]
Now convert this fraction into a percentage:
\[ \frac{21}{50} \times 100 = 42% \]
Therefore, the probability that a person spends their time going to the movies, given that they are 31–35 years old, is
\[ \boxed{42%} \]