[Answer] Review Exercises # 1&2

using System;
using System.Collections.Generic;
using System.Text;

namespace Albores_Francis
{
 class Program
 {
     static void Main(string[] args)
     {
         Console.WriteLine("------------------Problem 1------------------");
         string a, b, c, d;
         int sum, prod, diff, A, B, C, D;
         float qout;
         Console.Write("Enter integer 1: ");
   a = Console.ReadLine();
   Console.WriteLine("");
         Console.Write("Enter integer 2: ");
   b = Console.ReadLine();
   Console.WriteLine("");
         Console.Write("Enter integer 3: ");
   c = Console.ReadLine();
   Console.WriteLine("");
         Console.Write("Enter integer 4: ");
   d = Console.ReadLine();
   Console.WriteLine("");
         Console.WriteLine("");
   Console.WriteLine("");
         A = Int32.Parse(a);
         B = Int32.Parse(b);
         C = Int32.Parse(c);
         D = Int32.Parse(d);
         sum = A + B + C + D;
         prod = A * B * C * D;
         diff = A - B;
         qout = 1/2;
         Console.WriteLine("Sum: " + sum);
   Console.WriteLine("");
         Console.WriteLine("Product: " + prod);
   Console.WriteLine("");
         Console.WriteLine("Difference (1 and 2): " + diff);
   Console.WriteLine("");
         Console.WriteLine("Quotient (3 and 4): " + qout);
   Console.WriteLine("");

         Console.WriteLine("------------------Problem 2------------------");
         string numofStud_S, grade_S, persentPS, persentFS;
         int numofStud, grade, passed = 0, failed = 0;
         float persentP, persentF;
         Console.Write("Enter number of students to process: ");
         numofStud_S = Console.ReadLine();
         numofStud = Int32.Parse(numofStud_S);
    bool invalid = false;
   for (int x = 0; x < numofStud; x++)                                       
          {
   do                              
   {                                        
    Console.Write("Enter grade for student {0} 60-100: ", x + 1);                      
    grade_S = Console.ReadLine();                      
    grade = Int32.Parse(grade_S);                      
    if (grade <> 100)
                    {
                         Console.WriteLine("Error: Grade must be within 60 to 100");
                         invalid = true;
                    }
                    else
                        invalid = false;
               } while (invalid == true);
               if (grade >= 75 && grade <= 100)
    passed++;                 
   else                 
   {                      
    failed++;                   
   }              
  }              
  persentPS = passed.ToString();         
  persentFS = failed.ToString();              
  Console.WriteLine("");              
  Console.WriteLine("Total no. of students: "+numofStud);              
  Console.WriteLine("No. of students who passed: " + passed);              
  Console.WriteLine("No. of students who failed: " + failed);   
  Console.WriteLine("");              
  persentP = (float.Parse(persentPS) / float.Parse(numofStud_S) * 100);              
  persentF = (float.Parse(persentFS)/float.Parse(numofStud_S)*100);               
  Console.WriteLine("Percentage of students who passed: "+persentP+"%");              
  Console.WriteLine("Percentage of students who failed: " + persentF + "%");              
  Console.ReadLine();        
 }      
   }   }