The table gives some values of possible lengths x, in miles, of a monorail system, and their corresponding costs to build f(x), in millions of dollars. Which of the following equations models this relationship?

The correct answer is:

We use the formula,
Explanations:
Step 1 of 2:
To build 5 miles of a monorail system, it costs 405 million of dollars, then per length cost is million of dollars.
For 10 miles, it costs 810 millions of dollars, then per length cost is millions of dollars.
For 15 miles, it costs 1215 millions of dollars, then per length cost is millions of dollars.
Step 2 of 2:
We observe that for the length of miles to build, it costs 81 times the length of the monorail system.
Hence, the equation models the relationship is given by,
, where x is the length, in miles, of a monorail system and f(x) is their corresponding costs to build, in millions of dollars.
Final Answer:
The right choice is— f(x) = 81x.