CREATE TABLE table_204_131 (id DECIMAL, "tops design code" TEXT, "electrical system" TEXT, "max speed" TEXT, "weight" TEXT, "brakes" TEXT, "route availability" DECIMAL, "notes" TEXT)

how many have a max speed of 20 but are only dual brakes ?
SELECT COUNT(*) FROM table_204_131 WHERE "max speed" = 20 AND "brakes" = 'dual'
SELECT COUNT(*) FROM "table_204_131" WHERE "brakes" = 'dual' AND "max speed" = 20