Answer
$2$
Work Step by Step
We know that for an isothermal process
$P_iV_i=P_fV_f$
$\implies PiV_i=P_f(2V_i)$ Given that $V_f=2V_i$
This simplifies to:
$\frac{P_i}{P_f}=2$
$\implies P_f=\frac{P_i}{2}$
Thus, the pressure of the gas is decreased by a factor of 2.