A periodic cost is something you pay for over and over again instead of
just once. This is for things like a water bill or a magazine
subscription. Usually you pay either once per month or once per year.
It can be useful to convert between a monthly cost and a yearly cost to
compare two periodic costs. For example, a magazine subscription is $10
per month. To calculate how much you would pay every year, do the
following:
monthly cost = $10
yearly cost = $10 × 12 = $120
Now let's say that magazine subscription is $90 per year. To calculate
how much you would pay every month, do the following:
yearly cost = $90
monthly cost = $90 ÷ 12 = $7.50