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 Difficulty by Sir Percivale)

If you want to see what a real D4 problem looks like, see David Shin's "Remainder of One" pid=2698.  It is illuminating to read all the comments as well as the solution.  David Shin was a 4th year Computer Science student at MIT at the time this problem was posted.  A professional mathematician is someone with a PhD in mathematics (not computer science, physics, or engineering) whose career is entirely devoted to mathematics (usually as a professor). People who are good at puzzle math usually are well-versed in arithmetic mod n which does not require a PhD in anything, just a little studying and some facility in manipulating expresssions.

Posted by Richard on 2006-06-29 19:37:36