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On July 1st:  You put in another $100.

$621.3535211 + $100 = $721.3535211

On the last day of July:

They compute your interest...

721.3535211 + .01( 721.3535211 ) = $728.5670563

On August 1st:  You put in another $100.

$728.5670563 + $100 = $828.5670563

On the last day of August:

They compute your interest...

828.5670563 + .01( 828.5670563 ) = $836.8527268

On September 1st:  You put in another $100.

$936.8527268

On the last day of September:

They compute your interest...

$946.2212541

On October 1st:  You put in another $100.

$1046.2212541

On the last day of October:  Interest

$1056.683467

On November 1st:  Another $100

$1156.683467

On the last day of November:  Interest

$1168.250301

On December 1st:  Another $100

$1268.250301

On the last day of December:  Interest

$1280.932804

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.