Part of understanding function notation in a word problem is reading carefully. Don't be intimidated or think this is over your head. It is not. Let me show you:
You are told:
The total cost f(x), in dollars...
so f(x) =100 means the cost in dollars is 100.
If given f(anything) = a number, that number is the cost in dollars. That is what the story tells you.
f(x) = 10 + 20x means they give you some equation to calculate the cost, since they tell you f(x) = cost..
Going on in the story...
for renting a bike for x hours
Here they tell you what x is. x is number of hours you rent the bike.
In the context of the story, if you rent a bike for 2 hours, x would be 2.
So
What does f(10) represent?
f(anything) = cost in dollars.
The 10 is sitting in the place of the x, and x is defined as the number of hours the bike is rented.
So the answer is
The number of dollars it costs to rent the bike for 10 hours
which likely should have the letter D next to it.
The key is reading the story carefully, processing as you go along.
