"Our evidence indicates that at this period the mint ratio of gold to silver was 1:6±. Thus one nomisma or solidus, being 1/72 of a pound of gold, was officially worth 1/12 of a pound of silver, and a pound of silver ought to have sold for twelve nomismata. In buying silver for two nomismata a pound, Germanus was acquiring it at 1/6 of its mint value; at three nomismata a pound, he was getting it at 1/4 of its mint ratio."

(http://www.autoadmit.com/thread.php?thread_id=5527213&forum_id=2#47651355)