I read from a book that the jth level (starting from j=0 or the root) of a binary tree with n nodes divides a problem into $2^j$ subproblems, each of size $\frac{n}{2^j}$. I understand where $2^j$ comes from, but where does $\frac{n}{2^j}$ come from?
$n$ includes nodes above the level, right? Hence, how come the sum of the sizes of the subproblems of the subtrees of the level is $n$? Shouldn't it be less than $n$?