Maximise $Z=5x_{1}+3x_{2}$

subject to

$\begin{array}{} x_{1}+2x_{2} \leq 10, \\ x_{1}-x_{2} \leq 8, \\ x_{1}, x_{2} \geq 0 \end{array}$

In the starting Simplex tableau, $x_{1}$ and $x_{2}$ are non-basic variables and the value of $Z$ is zero. The value of $Z$ in the next Simplex tableau is _______.