Answer:
The given sequence shows that Francisco goes to the gym 5 days per week (since each term in the sequence increases by 5).
In mathematical terms, this represents a linear relationship between the week number and the total number of days Francisco has used his gym membership, which can be expressed as a function.
The function that describes this pattern is:
f(x) = 5x - 1
where
x is the week number and
f(x) is the total number of days Francisco has used his gym membership at the end of the week.
This function works because when you plug in 1 for the first week, you get 4 days (5*1 - 1 = 4), and when you plug in 2 for the second week, you get 9 days (5*2 - 1 = 9), and so on, which corresponds to the given sequence.