Find partial sums of the series $12+105+1008+10011+\dots$ 
Find the sum of $n$ terms of this series-
  $$12+105+1008+10011+.....$$

I did not understand that how should I proceed with this problem.

Hint: $$12=10^1+3\cdot1-1$$ $$105=10^2+3\cdot2-1$$ $$1008=10^3+3\cdot3-1$$ $$100011=10^4+3\cdot4-1$$
