#include <stdio.h>
#include <math.h>
/* Define doubling_time first */
double doubling_time (double r) {
  return log (2.0) / log (1.0 + r / 100.0);
}
/* Then, define main */
main ( ) {
  double r = 5.0;
  printf ("The value of r is %f.\n", r);
  printf ("The doubling time at twice that rate is %f.\n",
          doubling_time (r + r));
  printf ("The value of r is still %f.\n", r);
}