An industrial oven is programmed so that the humidity y (in \%) should decrease with the oven temperature x (in ∘C) at a constant rate of dxdy=−0.15%∘C−1 when the temperature is around 180∘C. At one moment the temperature is rising at 1.2∘C min−1.
a. (i) Determine the rate at which the humidity should be changing with respect to time at that moment, expressing your answer in %min−1.
b. (ii) Comment on the physical meaning of the sign of your answer.