CREATE TABLE table_66670 ("Name" TEXT, "Employed" TEXT, "Position held" TEXT, "Honour" TEXT, "Citation" TEXT)

What are the years employed shown for mathematics & prefect master?
SELECT "Employed" FROM table_66670 WHERE "Position held" = 'mathematics & prefect master'
SELECT "Employed" FROM "table_66670" WHERE "Position held" = 'mathematics & prefect master'