To set up an equation for this scenario, let's define \( x \) as the number of ride tickets Penelope buys.
- The entry cost to the carnival is $20.
- Each ride ticket costs $0.50.
- Penelope spends a total of $35.
The total cost can be represented by the equation:
\[ 20 + 0.50x = 35 \]
This equation states that the $20 for entry plus the $0.50 for each ticket multiplied by the number of tickets \( x \) equals the total amount spent, which is $35.
So, the correct equation from the options provided would be:
\[ 0.50x + 20 = 35 \]