user: Question: Which Effic is the highest one that has an Avg/G smaller than 305.6, and a GP-GS of 13-13?
Columns: ['Name', 'GP-GS', 'Effic', 'Cmp-Att-Int', 'Avg/G']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['Reesing, Todd', '13-13', 148.52, '329-495-13', 299.1]
SQL:

assistant: SELECT MAX("Effic") FROM "Table" WHERE "Avg/G" < 305.6 AND "GP-GS" = '13-13';