Practice code c language

#include <stdio.h>
int main()
{
    int num1,num2, value;
    char sing;
    
    printf ("please inter a number ");
    scanf ("%d",&num1);
    printf ("please inter another number : ");
    scanf("%d",&num2);
    value = num1+num2;
    
    return 0;
}

#include<stdio.h> // include stdio.h

int main()
{
    float marks;
    char grade;

    printf("Enter marks: ");
    scanf("%f", &marks);

    if(marks >= 90)
    {
        printf("A");
        grade = '+';

    }
    else if(marks >= 80 && marks < 90)
    {
        grade = 'A';
    }
    else if(marks >= 70 && marks < 80)
    {
        grade = 'B';
    }
    else if(marks >= 60 && marks < 70)
    {
        grade = 'C';
    }
    else if(marks >= 50 && marks < 60)
    {
        grade = 'D';
    }
    else if(marks >= 40 && marks < 50)
    {
        grade = 'E';
    }
    else
    {
        grade = 'F';
    }

    printf("Your grade is %c", grade);

    return 0;
}

#include <stdio.h>
int main ()

{
    double a,b,sum;
    scanf("%lf",&a);
    scanf("%lf",&b);
    sum=a+b;
    printf("sum is:%d\n",sum);
    return 0;
}

#include <stdio.h>
int main ()
{
    double a,b,sum;
    a=9.5;
    b=8.743;
    sum=a+b;
    printf ("sum is:%lf\n",sum);
    printf ("sum is;%0.10lf\n",sum);
    return 0;
}

#include <stdio.h>
struct student {
    char firstName[60];
    int roll;
    float marks;
} s[60];

int main() {
    int i;
    int n;

    printf("Enter number of student " );
    scanf("%d", &n);
    printf("Enter information of students:\n");


    // storing information
    for (i = 0; i < n; ++i) {

        s[i].roll = i + 1;

        printf("\nFor roll number%d,\n", s[i].roll);
        printf("Enter first name: ");
        scanf("%s", s[i].firstName);
        printf("Enter marks: ");
        scanf("%f", &s[i].marks);

    }

    printf("\n\n Displaying Information:\n\n");

    // displaying information
    for (i = 0; i < n; ++i) {
        printf("\nRoll number: %d\n", i + 1);
        printf("First name: ");
        puts(s[i].firstName);
        printf("Marks: %.1f", s[i].marks);
        printf("  ");

        if(s[i].marks <= 34){
            printf("Disqulified\n");

        }else if(s[i].marks >= 35 && s[i].marks < 100 ){

            printf("Qulified\n");
        }
    }

    return 0;
}