CREATE TABLE "railbuses" (
  "class" text,
  "railway_number_s" text,
  "quantity" real,
  "year_s_of_manufacture" text,
  "axle_arrangement_uic" text
);

Question: what is the quantity when the year of manufacture is 1900 and the class is mc?
SQL: SELECT "quantity" FROM "railbuses" WHERE "year_s_of_manufacture"='1900' AND "class"='mc';

Question: what is the class when the quantity is 4?
SQL: SELECT "class" FROM "railbuses" WHERE "quantity"=4;