Systems of equations for video game purchases

A store sells used and new video games.  New video games cost more than used video games.  All used video games cost the same.  All new video games cost the same.

Omar spent a total of $84 on 4 used video games and 2 new video games.  Sally spent a total of $78 on 6 used video games and 1 new video game.  Janet has $120 to spend.

Find the number of used video games Janet can purchase after she purchases 3 new video games.

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