This code gets WA 1:
Locale.setDefault(Locale.US);
...
out.printf("Mean = %.2f\n", (double)bestSum / total);

And this one gets AC:
Locale.setDefault(Locale.US);
...
StringWriter sw = new StringWriter();
PrintWriter pw = new PrintWriter(sw);
pw.printf("Mean = %.2f", (double)bestSum / total);
pw.flush();
String str = sw.getBuffer().toString();
pw.close();
out.println(str);

Oh, I've found the reason: their checker is bad and you should use "Mean = %.2f\r\n" (with '\r'!), not "Mean = %.2f\n".

Edited by author 09.07.2012 16:19

surprisingly this is the case.