Hiroe - I tried some things in my average application to try and replicate your error.
This sounds rediculous (but it HAS happened to me) - do you have the file named Average, but have it declared as "public class average" (noting the uppercase A and lowercase a). Double check this. When I changed my "public class average" to have an uppercase A, I was able to replicate your error.
Hope this helps - let me know if not, and I'd be happy to try and think of more options! : )
Loreta Anderson
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Life - no argument to win, just understanding to be earned.