91: Mainline

Of course, if you propose to use the approximation formula many times, you certainly should define a function, perhaps named doubling_time, to do the work:

#include  
// Definition of doubling_time belongs here 
main ( ) { 
  printf ("Your money doubles in about %f years at %f percent.\n", 
                                       doubling_time (4.0), 
                                                   4.0); 
}