if u think a number between 10 n 19 u alwas get 9
if u think a number between 20 n 29 u alwas get 18
if u think a number between 30 n 39 u alwas get 27
like wise u only get these numbers...9,18,27,36,45,54,63,72,81...
so it's not that difficult.....hehe....