How many engines were built with a cylinder size of 20 ½” x 26”, firebox is belpaire and valve gear is from Stephenson?
CREATE TABLE "engine_num7002" (
  "class" text,
  "firebox" text,
  "cylinder_size" text,
  "valves" text,
  "valve_gear" text,
  "number_built" text,
  "years_built" text
);
SELECT "number_built" FROM "engine_num7002" WHERE "cylinder_size"='20 ½” x 26”' AND "firebox"='Belpaire' AND "valve_gear"='Stephenson';