The message
from Roger 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.

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

Start with 100.

20% of 100 = 20, leaving 80. 8% of 80 = 6.4%, leaving 73.6

8% of 100 - 8, leaving 92. 8% of 92 = 7.36, leaving 84.64

--
Skipweasel
Pay no attention to that man behind the curtain.