This is phrased oddly, so I'll give two responses:
They exact way you have worded it is 2/20, which is 1/10, plus 10 from the 5+5 giving you 10.1
20 divided BY 2, not FROM 2 gives you 10, plus 5 plus 5 giving you 20. Usually something is divided by something else, hence this answer, but the way you phrased it you should use the above answer