Find the sum to the n terms:

Question

5+55+555+.......

Answer 1

1
1+11 = 12
1+11+111 = 123
So, for n terms, that ends up with
n*10^0 + (n-1)*10 + (n-2)*n^2

see whether you can arrive at the solution here:

http://www.wolframalpha.com/input/?i=sum(k%3D0..n-1)+(n-k)*10%5Ek