The digital root of a number is obtained by summing its digits and then repeating the process (summing the sum's digits) until the answer is a single digit number. For example, the digital root of 7368 is 6.

Determine the digital root of (9⁷²⁴⁰+38)⁸⁴⁵⁷

(In reply to very simplistic explanation by John)

In addition to the formula in terms of the post, it also follows from Raman's Formula that the digital root of any positive integer ( that is not divisible by 9) is equal to the remainder obtained by dividing the said number by 9.
Digital root of any positive integer divisible by 9 is always 9. 
REFERENCES: 
(i) http://en.wikipedia.org/wiki/Digital_root ;
(ii)http://mathworld.wolfram.com/DigitalRoot.html.

Posted by K Sengupta on 2006-07-11 14:49:27