The maximum theoretical efficiency of the system is the one corresponding to the efficiency of a Carnot cycle operating between the same temperatures of the system:
[tex]\eta=1- \frac{T_c}{T_h} [/tex]
where [tex]T_c[/tex] and [tex]T_h[/tex] are the cold and hot temperatures, respectively.
In our problem, [tex]T_c=300 K[/tex] and [tex]T_h=450 K[/tex], therefore the maximum theoretical efficiency is
[tex]\eta=1- \frac{300 K}{450 K}=0.33 [/tex]
So, 33%.
