Provide solutions for each of the following:

1.  What would the find command be to find all ordinary files modified within the last 2 
    hours that have an extension of txt?
    
2.  What is the command to add read permissions for everyone to a file?

3.  Write a program which outputs the prime numbers less than 100.  It is useful to note 
    that if a number is not prime, its largest factor is at most the square root of the 
    number.
    
4.  Using printf and for loops, print a pyramid consisting of '*' characters, 10 levels     
    high.  It should have twice the number of '*' as the level number, starting with 
    level 1 at the top, and should look like

             **
            ****
           ******
          ********
         **********
        ************
       **************
      ****************
     ******************
    ********************

5.  Write a loop which outputs the sum of every 3rd odd number less than 1000, in 
    decreasing order.  In other words, what is the sum of 999 + 993 + 987 + 981 + ...