When  87 mpg-e (39kw-hrs/100mi) is the  epa rated combined fuel economy how many operating modes are there?
create table table_24620684_2 (operating_mode varchar, epa_rated_combined_fuel_economy varchar, PRIMARY KEY (operating_mode))
select count(operating_mode) from table_24620684_2 where epa_rated_combined_fuel_economy = "87 mpg-e (39kw-hrs/100mi)"