How it works
In the made-up town of this story, cousins Cass and Cal each got a bill of ten coins with a ringed due day.
Cass paid all ten before the due day. That is paying in full: nothing is left and nothing extra is charged. The days before the due day usually cost nothing.
Cal paid five of the ten. The five still owed are a balance, and the bank adds one extra coin for every five still owed, so next month he owed six.
If the full bill is too big, tell the bank early. A plan made early is kinder than silence, for everyone.