You had a bunch of errors in there, 19 to be exact from the start. So I cleaned up the code a bit and fixed the errors, as well as added some much needed formatting
Look at what you had before and what I've done and see if you can find the differences and why I put them there. Take note that I didn't check to see if the program works as planned, that's your job. I just made sure that it compiles.
Code:
# include <stdio.h>
int main(void){
/* Variable declarations: */
int idno, test1, test2, final_test,total_number_of_A = 0,total_number_of_B = 0,
total_number_of_C = 0, total_number_of_D = 0, total_number_of_F = 0;
double semester_average = 0.0, class_average = 0.0;
int number_of_students = 0;
/* Function body: */
printf ("***> Enter student idno:");
scanf ("%d", & idno);
printf ("***> Enter test1:");
scanf ("%d", & test1);
printf ("***> Enter test2:");
scanf ("%d", & test2);
printf ("***> Enter final_test:");
scanf ("%d", & final_test);
while (idno != 0) {
semester_average=0.20*test1+0.30*test2+0.50*final_test;
printf ("The semester average is %lf\n", semester_average);
if (semester_average >= 90 && semester_average <= 100){
printf ("The letter grade is A\n");
total_number_of_A = total_number_of_A+1;
}
if (semester_average >= 80 && semester_average <= 89){
printf ("The letter grade is B\n");
total_number_of_B = total_number_of_B+1;
}
if (semester_average >= 70 && semester_average<= 79){
printf ("The letter grade is C\n");
total_number_of_C = total_number_of_C+1;
}
if (semester_average >= 60 && semester_average <= 69){
printf ("The letter grade is D\n");
total_number_of_D = total_number_of_D+1;
}
if (semester_average >= 0 && semester_average <= 59){
printf ("The letter grade is F\n");
total_number_of_F = total_number_of_F+1;
}/* end if */
printf ("***> Enter student idno:");
scanf ("%d", & idno);
if(idno == 0){ break; }
printf ("***> Enter test1:");
scanf ("%d", & test1);
printf ("***> Enter test2:");
scanf ("%d", & test2);
printf ("***> Enter final_test:");
scanf ("%d", & final_test);
} /* end while */
/*Calculate Class Average*/
class_average = ((4 * total_number_of_A) + (3 * total_number_of_B) +
(2 * total_number_of_C) + (1 * total_number_of_D))/
number_of_students;
printf("Grade distribution:");
total_number_of_A = total_number_of_A + 1; printf("%d", total_number_of_A);
total_number_of_B = total_number_of_B + 1; printf("%d", total_number_of_B);
total_number_of_C = total_number_of_C + 1; printf("%d", total_number_of_C);
total_number_of_D = total_number_of_D + 1; printf("%d", total_number_of_D);
total_number_of_F = total_number_of_F + 1; printf("%d", total_number_of_F);
printf("Class average: %lf", class_average);
return 0;
} /* end function main */