The relationships are:
 20 large marshmallows = 3 cups cereal
 3 mini marshmallows = 1 large marshmallow
  
 Therefore if there are 6 cups cereal, so we also get 40 large marshmallows. Since for every 1 large, we need 3 mini, hence:
 mini marshmallows = 40 * 3 = 120