When 380 g/mi (236 g/km) is the dirty electric grid rocky mountains (denver) what is the u.s national average electric mix?
create table table_24620684_2 (us_national_average_electric_mix varchar, dirty_electric_grid_rocky_mountains__denver_ varchar, PRIMARY KEY (us_national_average_electric_mix))
select us_national_average_electric_mix from table_24620684_2 where dirty_electric_grid_rocky_mountains__denver_ = "380 g/mi (236 g/km)"