### Video Transcript

Ruby spends three dollars and eighty-eight cents every day on transportation to
and from work. Write a function rule that relates the total amount of money Ruby spends on
transportation to the number of Ruby’s working days. Let 𝑥 represent the number of
working days and 𝑦 the total amount of money she spends on transportation.

We need to write a function rule. Our function rule will have an 𝑥 that represents the number of
working days and a 𝑦 that represents the total amount of money Ruby spends on
transportation. We can think of our function rule like a machine; you input some information that
you know, the function rule occurs, and then it gives you some kind of output.

In our case, we’ll input the number of working days; these are the days that Ruby
uses the transportation. After the function rule occurs in our machine, it will output the total amount
spent on transportation for those working days. The question is what happens here. What happens inside the function rule in our
problem? How do we go from the working days to the total money spent? And this is the key. The key is the three dollars and eighty-eight cents Ruby
spends on her working days.

Our function rule is the number of working days multiplied by three dollars and
eighty-eight cents and that gives us the total that Ruby spent. Our problem wants us to use 𝑥 to represent the number of working
days and 𝑦 to represent the total amount of money. So we substitute 𝑥 and 𝑦, respectively, for the working
days and the total amount. Simplifying it a little bit, three dollars and eighty-eight cents times 𝑥
equals 𝑦.
Using this function rule, we’ll always be able to find out the total amount of
money that Ruby spends on transportation if we know 𝑥 — how many days she worked.