Something I was pleased with was how easy it can be with strategies. Because not all strategies work for every problem. I really enjoyed learning R and C. Because it is quick and easy but it doesn't always work. Something I found hard was doubling and halving. Something that made me think was if I was getting it wrong because when you get something wrong you learn from you mistake. Something I want to get better at is how fast I am.
Here are some pictures of the the strategies. Here is a great post on this maths. http://parkvalemoriarty.blogspot.co.nz/2012/09/using-two-strategies.html