Answer:
Here is the C++ program:
#include <iostream>  //to use input output functions 
#include <fstream>  //to manipulate files
using namespace std;  //to identify objects like cin cout 
int main(){  //start of main function 
    ifstream file;   //creates an object of ifstream
    file.open("random.txt"); //open method to open random.txt file using object file of ifstream
        
     int numCount = 0;  //to store the number of all numbers in the file           
     double sum = 0;   //to store the sum of all numbers in the file 
     double average = 0.0;   //to store the average of all numbers in the file 
     int number ; //stores numbers in a file 
                  
         if(!file){   //if file could not be opened 
            cout<<"Error opening file!\n";    }   //displays this error message
        
        while(file>>number){   //reads each number from the file till the end of the file and stores into number variable 
            numCount++; //adds 1 to the count of numCount each time a number is read from the file           
            sum += number;  }  //adds all the numbers and stores the result in sum variable 
            average = sum/numCount;  //divides the computed sum of all numbers by the number of numbers in the file
      
        cout<<"The number of numbers in the file: "<<numCount<<endl;  //displays the number of numbers 
        cout<<"The sum of all the numbers in the file: "<<sum<<endl;  //displays the sum of all numbers
        cout<<"The average of all the numbers in the file: "<< average<<endl;  //displays the average of all numbers 
        file.close();     }  //closes the file    
     
Explanation:
Since the random.txt is not given to check the working of the above program, random.txt is created and some numbers are added to it:
35
48
21
56
74
93
88
109
150
16
while(file>>number) statement reads each number and stores it into number variable.
At first iteration:
35 is read and stored to number
numCount++;  becomes
numCount = numCount + 1
numCount = 1      
sum += number; this becomes:
sum = sum + number
sum = 0 + 35
sum = 35
At second iteration:
48 is read and stored to number
numCount++;  becomes
numCount = 1+ 1
numCount = 2    
sum += number; this becomes:
sum = sum + number
sum = 35 + 48
sum = 83
So at each iteration a number is read from file, the numCount increments to 1 at each iteration and the number is added to the sum.
At last iteration:
16 is read and stored to number
numCount++;  becomes
numCount = 9 + 1
numCount = 10    
sum += number; this becomes:
sum = sum + number
sum = 674 + 16
sum = 690
Now the loop breaks and the program moves to the statement:
        average = sum/numCount;  this becomes:
        average = 690/10;
        average = 69
So the entire output of the program is:
The number of numbers in the file: 10                                                                                                          
The sum of all the numbers in the file: 690                                                                                                    
The average of all the numbers in the file: 69
The screenshot of the program and its output is attached.