First, we note that 91 is not divisible by 2 or 3, so we can skip those prime numbers.
We then check if 91 is divisible by 5. Since 91 is not divisible by 5, we continue.
Next, we check if 91 is divisible by 7. Since 91 is divisible by 7, we can write 91 as $7 \cdot 13$.
The sum of the prime factors of 91 is $7 + 13 = \boxed{20}$.
The answer is: 20
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
What is the sum of the prime factors of 91?

### Answer:
