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Both(T&S) is zero because student cannot be in two classes.
Let us assume 100 students are there. If 70% are not second year students then 30 students are either thrid year or Neither
Since 40 students are third year then Neither = 30 because
T+Neither = 70 and T = 40
therefore S = 100-70 = 30
Total sudents who are not third year = 100-40 = 60
Students who are in second year = 30
so 30/60 = 1/2