Given a 2011 digit number, divisible by 9.
Let x be the sum of its digits and y the sum of the digits of x.
Find z, sum of the digits of y .
The sum of the digits of the first number has to be divisible by 9. If it is 2011 9's, then x=18099. Therefore, 9<=x<=18099. If x=9999, then y=36. Therefore, 9<=y<=36. The numbers 9, 18, 27, and 36 add up to 9. Therefore, z=9.

Posted by Math Man
on 20120302 20:48:59 