Table schema:
CREATE TABLE table_24620684_2 (us_national_average_electric_mix VARCHAR, dirty_electric_grid_rocky_mountains__denver_ VARCHAR)

Question: When 380 g/mi (236 g/km) is the dirty electric grid rocky mountains (denver) what is the u.s national average electric mix?
SQL: SELECT us_national_average_electric_mix FROM table_24620684_2 WHERE dirty_electric_grid_rocky_mountains__denver_ = "380 g/mi (236 g/km)"