What was the adult rate when the 16-17 year old rate was £3.40?
CREATE TABLE "current_and_past_rates" (
  "from" text,
  "adult_rate" text,
  "development_rate" text,
  "16_17_year_olds_rate" text,
  "apprentice_rate" text
);
SELECT "adult_rate" FROM "current_and_past_rates" WHERE "16_17_year_olds_rate"='£3.40';