Alright, lets get started.
Please refer the diagram I have attached.
Joyce rode her bicycle 4 miles from her house to the library.
Suppose the average speed of Joyce = x miles per hours
She rode the same distance from the library to the gym means 4 miles.
Her average speed from the library to the gym was 15 miles per hour less than her average speed from her house to the library
So, the average speed from library to the gym will be = [tex](x-15)[/tex] miles per hour
So this expression is same as (x-15), the expression Joyce modeled.
Hence (x-15) repreents Joyce's average speed from the library to the gym.
So option C is correct answer. : Answer
Hope it will help :)