I really shouldn't be allowed to have an abacus.
I always subtract AP (you owe it) and add AR (its owed you) but this is likely backwards.
AP you owe, but haven't paid, so you still have the money ... so you *add* it, AR you are owed, but haven't received so you subtract it.
Then you depreciate the paint, which confuses me.
edit: my thinking was that last Qs AP you have likely paid during this Q, and last Qs AR you have likely received by now, that's why I did it that way.