Final answer:
The relationship is a function because there is a specific amount of plant food remaining for each day. The function rule is f(x) = 72 - 12x, and the domain and range are {0, 1, 2, 3, 4, 5, 6} and {72, 60, 48, 36, 24, 12, 0}, respectively.
Step-by-step explanation:
The relationship between the amount of plant food remaining and the number of days is a function because for each day, there is exactly one unique amount of plant food remaining. This characteristic satisfies the definition of a function. Given that Ryan used the entire 72 milliliters of plant food over 6 days with a uniform decrease in the amount of plant food each day, the function rule for the amount of plant food remaining, f(x), after x days would be:
f(x) = 72 - 12x
Here, 12 is the amount of plant food used per day, which results from dividing the total amount of plant food (72 ml) by the number of days (6 days).
The domain of this function consists of the days Ryan observed the plant growth, which would be {0, 1, 2, 3, 4, 5, 6}, assuming day 0 is the starting point and day 6 is the end. The range reflects the amount of plant food remaining and would be {72, 60, 48, 36, 24, 12, 0}.