i don't have a clue what is wrong here....
the program is supposed to add, subtract, multiply and divide two real numbers.

#include<stdio.h>

float add(float x, float y);
float subtract(float x, float y);
float multiply(float x, float y);
float divide(float x, float y);

int main (void)
{

int choice;
float x, y;

while(1)
{
    printf("\nPress 1 to add two  numbers.\n");
    printf("Press 2 to subtract two numbers.\n");
    printf("Press 3 to multiply two numbers.\n");
    printf("Press 4 to divide two numbers.\n");
    printf("Press 5 to exit the program.\n");
    printf("\nEnter your choice:\n");
    scanf("%d",&choice);

    if(choice==5)
    {
        return 0;
    }

    if(choice >= 1 && choice <= 4)

    {
    printf("Please enter a real number:\n");
    scanf("%f", &x);

    printf("Please enter another real number:\n");
    scanf("%f", &y);

    if (choice==1)
    {    
    printf("The sum of both values inputted: % f\n", add(x, y));
    }

    float add(float x, float y)

    {
        return x+y;
    }

    else if (choice==2)
    {    
    printf("The difference of both values inputted: % f\n", subtract(x, y));
    }

    float subtract(float x, float y)

    {
        return x-y;
    }


    else if (choice==3)
        {    
    printf("The difference of both values inputted: % f\n", multiply(x, y));
    }

    float multiply(float x, float y)

    {
        return x*y;
    }

    else if (choice==4)
    {    
    printf("The difference of both values inputted: % f\n", divide(x, y));
    scanf("%f",x/y)
    }

    float divide(float x, float y)

    {
        return x/y;
    }


}
}

what doesn't it do that you want it to do? Or what does it do that you don't want it to do?

Few problems here:

  1. Function definition inside another function (take all the function definitions outside the main function scope).
  2. Missing } (you open 4 curly brackets for: main, while, if, if and you close only 3 curly brackets).
  3. Missing ; after scanf (line 75)
  4. Is there any reason at all for that scanf (line 75)?

That to ignore the warning coming from scanf.