foo.cc: In function 'int main()':
foo.cc:13:14: warning: comparison of constant '20' with boolean expression is always true [-Wbool-compare]
        if(2<n<20)cout<<setprecision(1)<<fixed<<(n-2)*4.+21;
           ~~~^~~
foo.cc:13:12: warning: comparisons like 'X<=Y<=Z' do not have their mathematical meaning [-Wparentheses]
        if(2<n<20)cout<<setprecision(1)<<fixed<<(n-2)*4.+21;
           ~^~

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