Define a class Student with following details .
private members :- regno, name, marks and average marks of a class.
public -
input() - to accept all the values
display() - to display all data members on the screen
findavg() - to calculate average of class
showavg() - static function to display the average marks of class
Input details of 3 student , find their average marks and display it.

#include<iostream>
using namespace std;
class Student {
 string name;
 int regno;
 int mark;
 static int avg;
 public :
 void input(){
  cout<<"Enter the name ";
  getline(cin,name);
  cout<<"Enter the reg no ";
  cin>>regno;
  cout<<"Enter the marks ";
  cin>>mark;
  cin.ignore(1,'\n');
 }
 void display(){
  cout<< name<<"\t"<< regno;
  cout<<"\t"<< mark<< endl;
 }
 void findAvg(Student a[]) {
  int sum = 0;
  for(int i = 0; i<3; i++) {
   sum = sum+a[i].mark;
  }
  avg = sum/3;
 }
 static void showAvg() {
 cout<<"Avg = "<< avg<< endl;;
 }
};
int Student::avg = 0;
int main() {
 Student s[3];
 for(int i =0; i<3; i++) {
  s[i].input();
 }
 for(int i =0; i<3; i++) {
  s[i].display();
 }
 s[0].findAvg(s);
 Student::showAvg();
 return 0;
}