user: Question: What was the US national average electric mix rating for the vehicle that was rated 280 g/mi (174 g/km) in the Midwest?
Columns: ['Vehicle', 'EPA rated All-electric range', 'EPA rated combined fuel economy', 'Alaska ( Juneau )', 'California (San Francisco)', 'Mid-Atlantic South (Washington, D.C.)', 'U.S. national average electric mix', 'Southeast (Atlanta)', 'Midwest (Des Moines)', 'Rocky Mountains (Denver)']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Mitsubishi i-MiEV', 'mi (km)', '112 mpg-e (30kW-hrs/100 miles)', '80 g/mi (50 g/km)', '100 g/mi (62 g/km)', '160 g/mi (99 g/km)', '200 g/mi (124 g/km)', '230 g/mi (143 g/km)', '270 g/mi (168 g/km)', '290 g/mi (180 g/km)']
SQL:

assistant: SELECT "U.S. national average electric mix" FROM "Table" WHERE "Midwest (Des Moines)" = '280 g/mi (174 g/km)';