What is 15.5 units down? negative or positive?

It would be negative seeing the key word is down. (:
Hope I could help!

On a graph, going down or left is a decrease, while going up or right is an increase.
From the origin point, going down 15.5 units would result in negative 15.5 (-15.5).
Think about if you have 4 apples. If you add 2 more, that would an increase and would be represented by a positive number, for positive rate of change. But if you then take away 3 apples, then your amount of apples would go down, it would decrease. And this decrease is represented for a negative number, for a negative rate of change.

So the answer is negative.