How it works
In the made-up town of Lantern Bay, Kai earned ten pretend coins a month. A bicycle cost twelve, so Kai was two short, and Bea lent the twelve.
The plan was three coins a month for four months, and Bea asked one extra coin each month, so Kai paid four coins a month.
On payday four of the ten went to Bea and six stayed with Kai. A loan spends coins from months that have not happened yet, which is how it shrinks your future.
Kai paid sixteen coins for a bicycle that cost twelve. The extra four were the price of riding today. Odo saved instead and paid twelve, three months later.