What is the largest lead margin when Schweitzer has 55% and Jones is not an option?
CREATE TABLE "opinion_polling_for_the_united_states_gu" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_brian_schweitzer" text,
  "republican_roy_brown" text,
  "libertarian_stan_jones" text,
  "lead_margin" real
);
SELECT MAX("lead_margin") FROM "opinion_polling_for_the_united_states_gu" WHERE "democrat_brian_schweitzer"='55%' AND "libertarian_stan_jones"='not an option';