Avergaging elements in array - arrays

Question

I'm trying to add the elements in an array. It's just a simple program to calculate the average of student grades. I know this is probably a rudimentary way to code this, I'm looking to do it more efficiently. However my code is not returning the average. I would greatly appreciate any help. I did try this with a for loop but got the same incorrect answer.
#include <stdio.h>
int main()
{
int grades[6];
int average;
int sum = 0;
printf("Please enter your five test scores:\n");
scanf("%d", &grades[0]);
scanf("%d", &grades[1]);
scanf("%d", &grades[2]);
scanf("%d", &grades[3]);
scanf("%d", &grades[4]);
scanf("%d", &grades[5]);
sum = sum + grades[6];
average = sum / 5;
printf("The average of the students test scores is %d:\n", average);
return 0;
}

Answer 1

You should sum all grades and then divide by their amount (in your case it is 6 not 5, because grades array has 6 elements). Here is a code sample:
#include <stdio.h>
int main()
{
int grades[6];
int average;
int sum = 0;
printf("Please enter your six test scores:\n");
scanf("%d", &grades[0]);
scanf("%d", &grades[1]);
scanf("%d", &grades[2]);
scanf("%d", &grades[3]);
scanf("%d", &grades[4]);
scanf("%d", &grades[5]);
for (int i = 0; i < 6; i++)
sum = sum + grades[i];
average = sum / 6;
printf("The average of the students test scores is %d:\n", average);
return 0;
}

Answer 2

I'm trying to add the elements in an array.
This is maybe already a wrong track regarding the title (and the code) where it is about averaging elements in an array.
How you build the array is up to you; I choose the simplest way. An important decisions is: is size of array and number of values the same? That is what n_grades does. The four zeroes in the array initialization illustrate the difference.
An average most likely should be a floating point number.
A nasty problem with averages is the lurking overflow. Very unlikely in this setting, but there is a more robust (and elegant) algorithm. The (double) cast is the un-elegant part, but is needed because the division is between two integers. Still this is the compact core:
for (i = 0; i < n_grades; i++)
aver += (double) grades[i] / n_grades;
corresponding to the math formula:
i<n
A = Sum G_i/n
i=0
("Sum" is the big Sigma)
#include <stdio.h>
int main()
{
int grades[] = {10,10,9,10,11,11,0,0,0,0};
int n_grades = sizeof grades / sizeof*grades; // use all elements
//n_grades = 6; // use only first n elements
double aver = 0;
for (int i = 0; i < n_grades; i++)
aver += (double) grades[i] / n_grades;
printf("The average of the students test scores is %f (n= %d):\n",
aver, n_grades);
return 0;
}
Now it prints:
The average of the students test scores is 6.100000 (n= 10):
or, uncommented, i.e. limited to 6 grades::
The average of the students test scores is 10.166667 (n= 6):

Answer 3

You're assuming that grades[6] holds the sum of all the values in the grades array, which is wrong of course.
You need something like:
for (int i = 0; i < 6; i++)
sum = sum + grades[i];