1. The problem statement, all variables and given/known data Suppose C(r) is the total cost of paying off a car loan borrowed at an annual interest rate of r%. What are the units of C '(r)? What are the units of C'(r)? What is its sign? 2. Relevant equations C'(r)=dC/dr 3. The attempt at a solution So, I figured: C'(r) = dC/dr Which is dollars over interest rate(?)...which seems kinda awkward to me. I'm wondering if theres a better solution for what the units of C'(r) would be. (dollars per percent interest?) And I'm not sure what is meant by "What is it's sign"?