can some one help me with this program please- urgent

when i run it is giving me an error saying illegal use of floating point and cannot covert 'float(*)(float) 'to 'float'

this is the source code

Code:

#include <stdio.h>
#include <conio.h>


int discount (int x);
float tax (float y);


main ()
{


    int code[10];
   int i,after_discount;
   float after_tax,total;
   int cost=500;


   for (i=1;i<10;i++){
       printf ("Enter the book code");
      scanf ("%d", &code[i]);
      }


   total = 500.00 * 10;
   after_discount = total - discount (cost);
   after_tax = after_discount + tax (total);


   printf ("Discount is: %d\n", discount(cost));
   printf ("After discount: %d\n",  after_discount);
   printf ("Tax is : %.2f\n", tax (total));
   printf ("Amount to be paid: %.2f", after_tax);


   printf ("Book Codes\n");


   for (i=1;i<10;i++){


   printf ("\t%d", code[i]); }






getch ();
return 0;
}


int discount ( int x)
   {
   int four_books,discount;
    four_books = x * 4;
    discount = 0.10*four_books;
    return discount;
    }


float tax (float y)
    {
       tax = 0.10 *y;
      return tax;
      }

Code:

float tax (float y)
{
    float a_variable;
    a_variable = 0.10 *y;
    
    return a_variable;
}