Properties of numbers

1. If two digits number and a number obtained by reversing its digits are added then it will be divisible by 11.

     Ex-12+21=33,it is divisible by 11.
     
           45+54=99, it is divisible by 11.

       2. If two digits number and a number obtained by reversing its digits are subtracted then it will be divisible by 9.

      Ex-65-56=9, it is divisible by 9.

           93-39=54, it is divisible by 9.

        3. If two digits number and its sum of digits are subtracted then it will be divisible by 9.
      Ex-13-(1+3)=9, it is divisible by 9.

           29-(2+9)=18, it is divisible by 9.