Summation Summation Summation

Algebra Level 4

Given $x_k, y_k, z_k > 0$ for all $k$ satisfy the three summations:

$\begin{aligned} \displaystyle \sum_{k=1}^{2015} x_k & = & 50 \\ \displaystyle \sum_{k=1}^{2015} y_k & = & 169 \\ \displaystyle \sum_{k=1}^{2015} z_k & = & 961 \\ \end{aligned}$

What is the minimum value of $\displaystyle \sum_{k=1}^{2015} \left ( x_k y_k z_k \right )$ ?