This program will help you, try it out
using System;  
class SumOfOdds
{
  public static void Main()
 {
   int x=0, sumodd=0, sumeven=0, sumdiv7 = 0 ,totalno7 = 0, i;    
   for (i=0 ; i<=20 ; i++)
  {
   x = i % 2;
    if (x != 0)
   {
    sumodd = sumodd + i;
   }
    if (x == 0)
   {
    sumeven = sumeven + i;
   }
  }  
   //checking for the sum & no. of numbers divisible by 7    
  x = 0; // resetting the value of ''x''
   for (i=100; i<=200;i++)
  {
   x = i % 7;
    if (x == 0)
   {
    sumdiv7 = sumdiv7 + i;
    totalno7 = totalno7 + 1;
   }   
}     
  Console.WriteLine("Sum of all odd numbers from 1 - 20 = " + sumodd + "\n");
  Console.WriteLine("Sum of all even numbers from 1 - 20 = " + sumeven + "\n");
  Console.WriteLine("Sum of all numbers from 100 - 200, divisible by 7 = " +
sumdiv7 + "\n");
  Console.WriteLine("Total numbers from 100 - 200, divisible by 7  = " + totalno7 + "\n");
   Console.ReadLine();
 }
}