For the following chemical reaction at $300 \mathrm{K}$

$\mathrm{A} \stackrel{}{\rightleftharpoons }\mathrm{B} ∆\mathrm{H}=-2400 \mathrm{cal} {\mathrm{mol}}^{-1}$

The frequency factor for both forward and backward reactions is equal. ${\left[{\mathrm{K}}_{\mathrm{eq}}\right]}_{400\mathrm{K}}$ is $4{\mathrm{e}}^{-0.5\mathrm{n}}$ . What is the value of ‘$\mathrm{n}$’? 

(Assume $∆\mathrm{H}$ is independent of the temperature)