17. The sum of four consecutive odd positive integers is always 

 Solution:
              Odd number: A whole number that is not divisible by 2
           An odd number is an integer of the form n = 2k+1, where k is an integer.
              Examples: 1, 3, 5, 7, 9,.....

              The sum of four consecutive number is:  (2k+1) + (2k+3) + (2k+5) + (2k+7)
              (2k+1) + (2k+3) + (2k+5) + (2k+7) =  2k+1 + 2k + 3 + 2k + 5 + 2k + 7                     
              (2k+1) + (2k+3) + (2k+5) + (2k+7) =  8k + 1 + 3 + 5 + 7 
              (2k+1) + (2k+3) + (2k+5) + (2k+7) =  8k + 16
              (2k+1) + (2k+3) + (2k+5) + (2k+7) =  8(k + 2)
              8(k + 2) is a number divisible by  4 :  [8(k + 2)]/4 = 2(k + 2)