$\left( t _{1 / 2}\right)_{250 \text { torr }}=4\,min$
$t _{1 / 2} \propto a ^{1- n }$
As $t _{1 / 2}$ is independent of initial pressure. Hence, order is $1^{st}$ order.
$\left( {{\rm{R}} = 8.3\;{\rm{Jmo}}{{\rm{l}}^{ - 1}}{{\rm{K}}^{ - 1}},\ln \left( {\frac{2}{3}} \right) = 0.4,\left. {{e^{ - 3}} = 4.0} \right)} \right.$