Summing the previous values in an IEnumerable

Question

I have a sequence of numbers:

var seq = new List { 1, 3, 12, 19, 33 };

and I want to transform that into a new sequence where

Answer 1

var seq = new List { 1, 3, 12, 19, 33 }; 

for (int i = 1; i < seq.Count; i++)
{
   seq[i] += seq[i-1];
}