How to understand the analysis of expected running time of randomized quick-sort in this paper?
The root node corresponds to a subproblem of size $n$. This is larger than $(3/4)^1 n$ and at most $(3/4)^0 n$, so the root node lies in group $0$.

Given any element $x$ and any integer $i$, the expected number of subproblems of group $i$ containing $x$ is at most $2$. This is because if $P$ is a subproblem of group $i$ containing $x$ and $Q$ is its child subproblem containing $x$, then $Q$ is of group $i+1$ with probability at least $1/2$. (Some calculation is needed here.)

Linearity of expectation shows that the expected total size of all subproblems of group $i$ is at most $2n$ &mdash; we get this by summing the expected number of subproblems of group $i$ containing $x$ over all possible elements. (You can only understand this if you know what linearity of expectation is.)