What is the mean year of marriage when her age was more than 19 and his age was 30?
CREATE TABLE "chart_the_eight_wives" (
  "year_of_marriage" real,
  "name" text,
  "her_age" real,
  "his_age" real,
  "num_of_children" real
);
SELECT AVG("year_of_marriage") FROM "chart_the_eight_wives" WHERE "her_age">19 AND "his_age"=30;