accounting wise, if a value is positive, is an asset, you're above water, +50, you have 50 bucks in your account/pocket.
if a value is negative, is a liability, you're below water, -50, you owe 50 bucks, you're in the hole for 50 clams.
so Jackson owes Monica $15, he's in the hole for 15 bucks, namely his balance with his sister is -15.
if he pays her $1, he owes her 14, so -14 is his balance, let's make a quick table of that
pays her 1................... -14
pays her 2..................-13
pays her 3...................-12
and so on
pays her 13.................-2
pays her 15..................0
0 means, he paid her in full.
