Year	Number
1961	17.727272727272727
1962	19.090909090909093
1963	20.454545454545457
1964	21.818181818181817
1965	23.18181818181818
1966	24.545454545454547
1967	25.675675675675674
1968	27.027027027027028
1969	28.18181818181818
1970	28.636363636363637