Answer
$3\frac{1}{8}$ cubic meters
Work Step by Step
Let $P$ be the pressure.
Let $V$ be the volume.
$P=\frac{k}{V}$
$P=1250; V=2$
$1250=\frac{k}{2}$
$k=2500$
$P=\frac{2500}{V}$
$P=800$
$800=\frac{2500}{V}$
$V=\frac{2500}{800}$
$V=\frac{25}{8}=3\frac{1}{8}$ cubic meters