Ordering chicken nuggets, impossible NFL scores, and optimal change

It's just been pointed out to me that in the US, McDonalds sells Chicken McNuggets(TM) in denominations: 6, 10, 20 nuggets, and not 6, 9, 20 (which is how they do in the UK, where I am from).

Even though I now live in the USA, it appears I've never been into McDonalds and had need to note the sizes these items are sold in. I will not comment further on this :)