Which Effic is the highest one that has an Avg/G smaller than 305.6, and a GP-GS of 13-13?
CREATE TABLE "passing" (
  "name" text,
  "gp_gs" text,
  "effic" real,
  "cmp_att_int" text,
  "avg_g" real
);
SELECT MAX("effic") FROM "passing" WHERE "avg_g"<305.6 AND "gp_gs"='13-13';