Must a function that is decreasing over a given interval always be negative over the same interval?

No, the function doesn't have to be negative

Explanation:

We could have a function that is entirely positive on some interval where it is decreasing. The only requirement for a decreasing interval is that the function goes downhill as you move from left to right.

For example, the graph of y = x^2 is decreasing on the interval -10 < x < -3 but each y = f(x) value is above the x axis for this interval.

step-by-step explanation:

milk=16 large

soda=280 kids

juice box=82 regular

63 soda +63 regular

124 large total