calculating gpa with only grades?

is there some sort of calculation for just GPA without knowing the credits? i wrote a program which shows the average 20students have each after taking 3 exams each. now i must print the GPA of each student, as well as which has the highest GPA. i looked up some formulas via google search and didn't find much because the way GPA is calculated involves the credits taken.

Code:

`#include<stdio.h>`

int main()

{

float exam_a[20] = { 77, 65, 89, 79, 85, 90, 98, 45, 83, 75, 71, 85, 85, 80, 68, 72, 56, 82, 63, 80} ;

float exam_b[20] = { 87, 92, 84, 89, 75, 99, 92, 66, 80, 77, 56, 79, 76, 78, 88, 78, 66, 70, 69, 90} ;

float exam_c[20] = { 45, 77, 87, 77, 88, 83, 94, 71, 73, 83, 52, 80, 93, 88, 76, 64, 75, 80, 85, 52} ;

int k;

char letter;

float student[20];

float sum_a = 0, sum_b = 0, sum_c = 0, sum_d = 0, sum_f = 0;

int tot_a = 0, tot_b = 0, tot_c = 0, tot_d = 0, tot_f = 0;

for (k = 0; k < 20; ++k){

sum_a = sum_a + exam_a[k];

student[k] = exam_a[k] + exam_b[k] + exam_c[k];

student[k] = (student[k])/3;

if (student[k] >= 90){

letter = 'A';

++tot_a;

}

else if (student[k] >= 80){

letter = 'B';

++tot_b;

}

else if (student[k] >= 70) {

letter = 'C';

++tot_c;

}

else if (student[k] >= 60) {

letter = 'D';

++tot_d;

}

else {

letter = 'F';

++tot_f;

}

}

printf("(Based on Averages)Number of As = %i \number of Bs = %i \number of Cs = %i \number of Ds = %i \number of Fs = %i \n", tot_a, tot_b, tot_c, tot_d, tot_f);

}

is to show which students got what grades on the average of 3 exams. i was thinking of maybe just doing if 95 and over store 4.0 90 and over store 3.7 and so on.. unless theres a better way to calculate this.