From Boyle's Law,

${\mathrm{P}}_2{\mathrm{V}}_2={\mathrm{P}}_1{\mathrm{V}}_1$ 

${\mathrm{P}}_{1 }=940.3$ mm of $\mathrm{Hg}$

${\mathrm{P}}_{2 }=$ Pressure at which volume is reduced by $40\%$

${\mathrm{V}}_{2 }=$ Volume decreased by $40\%$

$\left({\mathrm{V}}_2={\mathrm{V}}_1-0.4 {\mathrm{V}}_1=0.6 {\mathrm{V}}_1\right)$

Then,${\mathrm{P}}_2=\frac{940.3\times {\mathrm{V}}_1}{0.6{\mathrm{V}}_1}$

$=1567.16$

$\approx 1567 \mathrm{mm} \mathrm{Hg}$