Year	Number
1961	40
1962	40
1963	40
1964	40
1965	40
1966	40
1967	40
1968	40
1969	50
1970	50
1971	50
1972	50
1973	50
1974	50
1975	50
1976	50
1977	60
1978	60
1979	60
1980	60
1981	60
1982	60
1983	60
1984	60
1985	60
1986	60
1987	80
1988	80
1989	80
1990	80
1991	80
1992	80
1993	80
1994	80
1995	80
1996	80
1997	80
1998	80
1999	80
2000	80
2001	80
2002	80
2003	80
2004	60
2005	60
2006	46.15384615384615