Answer
$\text{noticeable amount of water is left in it}$
Work Step by Step
$P_2$ = $P_1$ + $\rho$gh
$\text{But tank is closed,}$
$\therefore$ $\text{$P_1$ = 0}$
$P_2$ = $\text{0}$ + $\rho$gh
$P_2$ = $\rho$gh
$\text{$P_2$ = 1000$\times$9.8$\times$15}$
$P_2$ = 147000 Pa
$\text{After a valve is then opened at the bottom of the tank}$
$P_1$ = 101000 Pa
$P_2$ - $P_1$ = $\rho$gh
($P_2$ - $P_1$)/$\rho$g= h
h= (147000-101000)/(1000$\times$9.8)
h=4.69 m
$\text{So, noticeable amount of water is left in it}$
