Dr, Sun, can you give us the solution of exercise 4?

Harry_520 - while I did not get the solution right during the exercise - i did come up with a solution after thinking about the problem a bit longer.

you can solve the sum of digits by using a loop and modulo and division.  the basic idea it to isolate each digit and then add that digit to a result variable then do it all over again with the remaining digits.  i obviously can't post the exact solution, but hopefully this logic helps.