Consider the following linear programming problem:

Max $z=16x_1+13x_2$

$s.t.20x_1+10x_{2}60(constraintI)$

$,x_1+2x_{2}6(constraintII)$

$,x_1,x_{2}0.$

The constraint lines and optimal solution are illustrated in the following figure:

What is the lower bound of the $x_2$ coefficient $(C_2)$ in the objective function, that doesn't change the optimal solution?

$($Round to one decimal place.$)$