I wouldn't know about trial exams, but I can explain it a bit further if it helps.
Think of it like a linear relationship, without the '
'. So something like
. As an example, you might not know the exact relationship between
and
, but you may know that if you double
, then
doubles, and if
triples then
triples, et cetera; they always increase or decrease together by the same factor. But since you don't know the exact relationship, you just write
.
You could also rearrange that to
. This is more intuitive, since it just says that 'the ratio of
and
is always the same', which is another way of saying that
is proportional to
. Of course, this has the slight disadvantage of causing division-by-zero issues, so we generally write
.
In order to work out the value of a proportionality constant, you need extra information. In the VCAA question, these conditions were given in various ways to allow you to determine
on different days. In our example of
, you'd simply need a point that the line passes through other than the origin. It's really just an unknown, so you need one equation to find it, in general.