I don't understand why the author says that following matrix equation solution, is a "mean solution"
see, find out $[R]$
$$[N][R] = [B]$$
where $[N]$ is a 3x4 matrix, $[R]$ is a 1x3 matrix and $[B]$ is a 1x4 matrix
the solution is the following
$$[R] = [[N]^T[N]]^{-1}[N^T][B]$$
the author says,
"Because [N] is not square a trick must be used to obtain a solution. Normally this technique yields a mean solution."
So, what does it mean the term "mean solution" ??
Reread yourself my friend. $N\in M_{4,3},R\in M_{3,1},B\in M_{4,1}$. Your solution is obtained using the Gauss approximation method ( least squares). cf. http://en.wikipedia.org/wiki/Linear_least_squares_%28mathematics%29