On July 1st: You put in another $100.
On the last day of July:
They compute your interest...
On August 1st: You put in another $100.
On the last day of August:
They compute your interest...
On September 1st: You put in another $100.
On the last day of September:
They compute your interest...
On October 1st: You put in another $100.
On the last day of October: Interest
On November 1st: Another $100
On the last day of November: Interest
On December 1st: Another $100
On the last day of December: Interest
So, if you invest $100 at the beginning of each month at 12% compounded monthly, at the end of the year, you'll have $1280.93.
Your total interest earned is $80.93.