The message
from Guy King contains these words:

It doesn't matter which discount you take off first.

100 100
-20 -10
80 90
-8 -18
72 72

8% of 80 isn't 8 so the answer isn't 72. 18% of 90 isn't 18 - so the
answer isn't 72 in the left column either.

It is (or was) a convention on Usnet to selectively snip and leave in
the relevant part of the previous post which, in this case, was that the
two percentages were 20% and 10%. 10% of 80 is indeed 8 and 20% 0f 90 is
18.

If you're going to demonstrate it properly you need to choose two
percentages - like 20% and 8% and apply them properly.

QED.

--
Roger Chapman