Final answer:
The revenue function for Grooming USA is R(x) = 25x, representing a direct relationship between the number of pets groomed and the revenue, increasing by $25 per additional pet groomed.
Step-by-step explanation:
The revenue function for a business like Grooming USA, which charges $25 for every pet groomed, can be defined as R(x) = 25x, where R(x) represents the revenue and x represents the number of pets groomed in a month. This linear function shows that the revenue increases by $25 for each additional pet that is groomed.