Step 1 :Given that $c$ and $d$ are inversely proportional and are both positive, the equation of proportionality is $c=\frac{5}{d}$.
Step 2 :In an inverse proportion, as one quantity increases, the other decreases.
Step 3 :So, if $d$ increases, $c$ should decrease. Similarly, if $c$ increases, $d$ should decrease.
Step 4 :Final Answer: a) If $d$ increases, $c$ \(\boxed{decreases}\). b) If $c$ increases, $d$ \(\boxed{decreases}\).