I don't mix formula either. Bottles need to come pre-mixed. That being said 20 months is way too old IMO for formula!!! I actually put in my contract that all children need to be weaned off by 16 months. No reason by that age why they can't get enough out of food. I have had a few parents who continue to give bottles at home. Whatever! Lol. Do what you want at home but here not allowed.