Cassy can pack 12 x 10 = <<12*10=120>>120 jars of jam in the 10 boxes.
She can also pack 10 x 30 = <<10*30=300>>300 jars of jam in the 30 boxes.
A total of 300 + 120 = <<300+120=420>>420 jars of jam can be packed in the boxes.
So, 500 - 420 = <<500-420=80>>80 jars of jam will be left unpacked.
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