These tests are similar to the above, except that they represent
the~percent difference between two values. That is, given the
values $5$~and $15$, we arrive at a~$200%$ increase. So
if~$p=200$, then we want $5r=15, r=3=\((15-5)/5\)+1$.
Similarly, if we go from~$10$ to~$5$, we have~$p=-100$
and~$10r=5$; then~$r=1/2=0.5$.
\ref{_rtop_} provides a way to negate the result using a
short-hand form for convenience and conciseness.