Array issue in C

I have an university project in C programming. I ran into a problem with the following task. My program should order numbers in two arrays. In the first array i must save (the biggest of every fifth element) and that is my problem. I am not sure how to make the loop which reads five elements compare them, take the biggest one, and then continue doing this with the other elements. I am hoping someone to help because I blocked.

#define A 100
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
void show(int x[], int nx); //функция за показване на масивите
float vavedi(int x[], int nx); //функция, чрез която ръчно въвеждаме числата и ги обработваме
FILE* readFile(char* fname); //функция която чете файл и представя съдържанието му като масиви
int main()
{
int call, a = 0, b = 0, mode = 0, i = 0;
int check = 0;
char fail[A];
char* menu[] = {
"PROGRAM STARTED!",
"Enter an option:",
"1 : Write the numbers.",
"2 : Choose from a file.",
"0 : Exit."
};
do {
for (i = 0; i < 5; i++)
printf("%s\n", menu[i]);
check=scanf("%d", &mode);
if (check != 1)
printf("ERROR! Try again!");
switch (mode)
{
case 1: {
//в случай 1 числата се въведждат от потребителя
call = vavedi(a, b);
break;
}
case 2: {
//в случай 2 потребителя използва съществуващ файл
printf("Enter the path of the file you want to open:\n");
scanf("%s", fail);
call = readFile(fail);
if (call == NULL) {
printf("The file doesn't exist! Try again!\n");
}
break;
}
case 0:
break;
default:
{
printf("ERROR! Try again!\n");
}
}
} while (mode != 0);
printf("\nThe program ended!\n");
return 0;
}
void show(int x[], int nx)
{
int k;
for (k = 0; k < nx; k++)
{
printf("\n Element[%d]= %d", k, x[k]);
}
}
float vavedi(int x[], int nx)
{
int call=0;
int enter=0;
int imin, max;
int b[A], c[A];
int i, j, j1, count;
j = 0;
do {
printf("\nCount of the elements:");
scanf("%d", &count);
if (count <= 0 || count > 100)
printf("Invalid input! Try again!\n");
} while (count <= 0 || count > 100);
for (i = 0; i < count; i++)
{
printf("\nEnter an element:");
scanf("%d", &c[i]);
} 
printf("\n");
return enter;
}

Update : Including header file mentioned by @pmg.

If i understand correctly, your problem is to find largest element for every five elements:

    #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <limits.h> // Put this after your header files
..
.. //Rest of the code.
..
for (i = 0; i < count; i++)
{
printf("\nEnter an element:");
scanf("%d", &c[i]);
} 
size_t size = ceil(count/5.0);
memset(b , INT_MIN , size);
for(int i = 0 ; i < count ; i++)
{
b[i/5] = b[i/5] > c[i] ? b[i/5] : c[i];
}
// Print b like this:
for(i = 0 ; i < size; i++)
printf("%d" , b[i] );       

Let's see how this works for array of size 9. array's indices will go from 0 to 8 . If I divide each of them by 5, I get 0 for i = 0 to 4 and 1 for i = 5 to 8. Now we can take max over elements in buckets of 5. You seem to be a beginner, hope this helps you in creating better understanding.

The other problem is the following:

{
int c[A], b[A];
int i = 0, j = 0, k = 0, num;
FILE* fp= NULL; //указател за файл
fp = fopen(fname, "r");
if (fp)
{
while (fscanf(fp, "%d", &c[i]) != EOF)
i++;
num=i;
size_t size = ceil(num/5.0);
memset(b , INT_MIN , size);
for(i = 0 ; i < num ; i++)
{
b[i/5] = b[i/5] > c[i] ? b[i/5] : c[i];
}
printf("\nARRAY1:\n");
for(i = 0 ; i < size; i++)
{
buble(b, i);
printf(" Element[%d]=%1d\n", i, b[i]);
}
printf("\nARRAY2:");
buble(c, num);
show(c, num);
printf("\n");
}
fclose(fp);
return fp;
}
It doesn't calculate the rigth way.
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