Converting sunlight to electricity with solar cells has an efficiency of 15%. It's possible to achieve a higher efficiency (though currently at higher cost) by using concentrated sunlight as the hot reservoir of a heat engine. Each dish in the figure concentrates sunlight on one side of a heat engine, producing a hot-reservoir temperature of 500 degrees Celsius. The cold reservoir, ambient air, is approximately 30 degrees Celsius. The actual working efficiency of this device is 30%. What is the theoretical maximum efficiency?