Break down an amount into units of dollars and cents.

#include <stdio.h>

void breakdown(double amount)
{
   size_t i;
   struct currency
   {
      int cents;
      const char *name;
   } Unit[] =
   {
      { 10000, "hundreds" },
      {  5000, "fifties"  },
      {  2000, "twenties" },
      {  1000, "tens"     },
      {   500, "fives"    },
      {   100, "ones"     },
      {    25, "quarters" },
      {    10, "dimes"    },
      {     5, "nickels"  },
      {     1, "pennies"  },
   };
   int cents = amount * 100 + 0.5; /* add 0.5 to round */
   printf("$%.2f : ", amount);
   for ( i = 0; i < sizeof Unit / sizeof *Unit; ++i )
   {
      int count = cents / Unit[i].cents;
      if ( count )
      {
         printf("%d %s, ", count, Unit[i].name);
      }
      cents %= Unit[i].cents;
   }
   putchar('\n');
}

int main()
{
   double cost = 20.21, tendered = 50.00, change = tendered - cost;
   breakdown(cost);
   breakdown(tendered);
   breakdown(change);
   return 0;
}

/* my output
$20.21 : 1 twenties, 2 dimes, 1 pennies,
$50.00 : 1 fifties,
$29.79 : 1 twenties, 1 fives, 4 ones, 3 quarters, 4 pennies,
*/
The article starter has earned a lot of community kudos, and such articles offer a bounty for quality replies.