Prices 1970
19.1
Prices 1971
17.4
Prices 1968
21.4
Prices 1969
20.3
Prices 1966
23.0
Prices 1967 – Not available
22.4
Prices 1963
25.8
Prices 1964
25.0
Prices 1965
23.9
Prices 1960
28.4
19.1
17.4
21.4
20.3
23.0
22.4
25.8
25.0
23.9
28.4