8.4

#include <iostream>
#include <conio.h>
#include <cmath>
#include <string>
 using namespace std;

int sum( int num)
{
   if (num < 10)
      return num;
   else
      return (num%10)+sum(num/10);
}

int main()
{
     setlocale(LC_ALL, "Russian");
     cout << sum(39); //
    getch();
    return 0;
}