This is a favourite with me as it is such an easy little trick to remember, which also makes addition easier. When I was thinking about how I do calculations in my head I realised that if I had to add nine I usually took two steps. Firstly, I added ten and secondly, I took one from the answer. But I then realised that I didn't always do this. For example when adding 9 to 40 I just add the 9 as the answer does not cross the next tens boundary. I fully recommend this approach, but it has to be recognised that there are plenty of other efficient ways to carry out mental calculations. Add 9 to 2-digit numbers