There is a proportionality between [[$ a $]], [[$ b $]], [[$ c $]] and [[$ d $]], which can be marked as [[$ \displaystyle\frac {a} {b} = \displaystyle\frac {c} {d} $]].

a) What kind of proportionality exists between [[$ a $]] and [[$ c $]]?

[[$ a $]] and [[$ c $]] are directly inversely proportional.

b) What kind of proportionality exists between [[$ b $]] and [[$ c $]]?

[[$ b $]] and [[$ c $]] are directly inversely proportional.