I don't know if you are still interested in this, but I was, as it says "maths problem" in the title
For future reference, if nothing else, the actual probability distributions for 1d6oe and 2d6oe (=DRN in the manual) are:
and for the original question,
In all expressions X=5n+r, with 1<=r<=5 (note limits!).
The first one is more or less trivial, the second is proven by induction in
n and the third follows from that by straightforward summation.