How many Attendances that has a Visitor of philadelphia on may 20?
CREATE TABLE "playoffs" (
  "date" text,
  "visitor" text,
  "score" text,
  "home" text,
  "decision" text,
  "attendance" real,
  "series" text
);
SELECT AVG("attendance") FROM "playoffs" WHERE "visitor"='philadelphia' AND "date"='may 20';