Which Effic is the average one that has an Avg/G larger than 3.7, and a GP-GS of 13, and a Cmp-Att-Int of 318-521-15?
CREATE TABLE "passing" (
  "name" text,
  "gp_gs" text,
  "effic" real,
  "cmp_att_int" text,
  "avg_g" real
);
SELECT AVG("effic") FROM "passing" WHERE "avg_g">3.7 AND "gp_gs"='13' AND "cmp_att_int"='318-521-15';