A possible common denominator are -> 21/35 and 20/35.
I just multiplied the denominators by each other, if I can't find another way.
I would multiply five times sevon, to get 35, and then do the same to the numorator, getting 21. Then I would multiply sevon by five, getting 35, and then do the same to the numorator, getting 20.
I hope this helps! XD