user: Question: What is the total number of Viewers with a Rank (Night) of n/a, and a Timeslot of 8:30 p.m.?
Columns: ['Episode', 'Air Date', 'Timeslot', 'Rating', 'Share', '18-49 (Rating/Share)', 'Viewers (m)', 'Rank (Timeslot)', 'Rank (Night)', 'Rank (Overall)']
Types: ['text', 'text', 'text', 'real', 'real', 'text', 'real', 'real', 'text', 'real']
Sample row: ['"Homecoming"', 'March 18, 2008', '10:35 P.M.', 6.7, 12.0, '2.9/8', 9.93, 1.0, '5', 21.0]
SQL:

assistant: SELECT SUM("Viewers (m)") FROM "Table" WHERE "Rank (Night)" = 'n/a' AND "Timeslot" = '8:30 p.m.';