Frank, you win the prize for perseverance.
The full answer is as follows:
It seems like it would be 80% of 30% which is 24%. However, the formula for Fast Adrian's profit is:
30% more than he bought it for => 1.3
80% of what he sold it for => 1.3 * 0.8 => 1.04
so the profit becomes
1.3 - 1.04 = 0.26
His cash input/output looked like this:
-100% Bought car
+130% Sold car to sucker
-104% Bought back from sucker (80% of 130%)
+100% Sold car at original price
----------
26% Final profit.
This is the hardest one yet!
You have an analogue clock whose face is numbered in a circle from 1 to 12, with 12 facing "north", 3 "east", 6 "south", and 9 "west".
You are allowed to draw two lines which go all the way across the clock face, and divide the numbers on the clock into 3 or 4 groups depending on if the lines intersect.
How can you draw the lines so that the numbers in each group add up to the same sum.