How it works
Maya wanted a bike that cost 440 pretend coins, and she had 400 in a jar. A jar adds nothing, so after two years it still held 400, forty short.
A savings account at the Coin House keeps coins safe and pays a small yearly extra, called interest. Maya could take her coins back any day.
The pretend rule: 1 extra coin for every 20, each year. So 20 earns 1, 40 earns 2, and 400 earns 20.
In year two the account held 420, which is twenty-one twenties, so 21 arrived. The extra earned extra.