Answer
$ 9.67\;m$
Work Step by Step
In the previous section of this problem, we have obtained the pressure at B, which is
$P_B=P_0-\rho g(h_1+d+h_2)$
Now,
$P_B\geq 0$
or, $[P_0-\rho g(h_1+d+h_2)]\geq 0$
or, $\rho g(h_1+d+h_2)\leq P_0$
or, $(h_1+d+h_2)\leq \frac{P_0}{\rho g}$
$(h_1)_{max}= \frac{P_0}{\rho g}-d-h_2$
Substituting the given values
$(h_1)_{max}= \frac{1\times10^{5}}{1000\times9.81}-0.12-0.40$
or, $(h_1)_{max}= 9.67\;m$
