Ok, was going to code this somehow, but never mind, it's a while since I was flamed properly...
DS can add a bit, i.e. he can say 1 and 5 is 6, if this follows on from dh/me saying 1 and 4 is 5 etc.
Is this:
a) completely normal for a 3.6 year old.
b) advanced.
c) My child was doing this at 18 months.
d) Sod off?