The efficiency of a Carnot engine (maximum possible efficiency) is given by η = 1 - (T_cold / T_hot). Temperatures must be in Kelvin. T_cold = 30+273 = 303 K. T_hot = 300+273 = 573 K. η = 1 - (303/573) ≈ 1 - 0.529 = 0.471, or 47.1%.