user: Question: What is the sum of Artist Steve Hepburn's Mintage?
Columns: ['Year', 'Artist', 'Composition', 'Mintage', 'Issue Price']
Types: ['real', 'text', 'text', 'real', 'text']
Sample row: [2007.0, 'Steve Hepburn', '99.99% pure gold', 200.0, '$5,999.95']
SQL:

assistant: SELECT SUM("Mintage") FROM "Table" WHERE "Artist" = 'Steve Hepburn';