Year	Number
1961	2.9962546816479403
1962	6.092592592592593
1963	7.352941176470589
1964	10.243636363636362
1965	10.830324909747292
1966	13.317857142857143
1967	15.901060070671377
1968	18.53146853146853
1969	22.06896551724138
1970	24.52758620689655
1971	27.958620689655174