Subtract 2nd term minus 1st term

 23 1/3 =  23 2/6 =  22 8/6
-20 1/2 = -20 3/6 = -20 3/6
---------------------------
                      2 5/6

Subtract 3rd term minus 2nd term

 26 1/6 =  26 1/6 =  25 7/6
-23 1/3 = -23 2/6 = -23 2/6
---------------------------
                      2 5/6

Subtract 4th term minus 3rd term

 29        28 6/6  
-26 1/6 = -26 1/6
-----------------
            2 5/6
 
So the common difference is 2 5/6

So we add 2 5/6 each time to get the next term:

 29
+ 2 5/6
-------
 31 5/6 = 5th term

 31 5/6  
+ 2 5/6
-------
 33 10/6 = 34 4/6 = 34 2/3 = 6th term

 34 2/3 =  34 4/6
+ 2 5/6   + 2 5/6
-----------------
           36 9/6 = 36 3/2 = 37 1/2 = 7th term.

Edwin