When i taught sonshine to add 2 digits, i had expected him to be able to calculate in his head too. Normally, i wouldn't have such expectations but this time i know my son is capable of doing so hence i decided to give him a little nudge.
Initially, he couldn't do it. So i pulled away for a few weeks until i realised, he couldn't do it because he didn't have enough practise on paper i.e. he didn't get enough visual on adding in 2 digits. Hence, i took out the ipad and worked 3-4 sums with him.
It worked like magic!
I gave him a couple of verbal sums, 19+19? 26+15? etc and
Straight away, he learnt how to add 2 digits- all his head. :)