The coefficient of linear expansion of iron is 1.0 × 10−5 per c◦ . the surface area of an iron
cube, with an edge length of 5.0 cm, will increase by what amount if it is heated from 10◦ c to
60◦ c

 The maximum possible efficiency, i.e the efficiency of a Carnot engine , is give by the ratio of the absolute temperatures of hot and cold reservoir. 
η_max = 1 - (T_c/T_h) 

For this engine: 
η_max = 1 - [ (20 +273)K/(600 + 273)K ] = 0.66 = 66% 

The actual efficiency of the engine is 30%, i.e. 
η = 0.3 ∙ 0.664 = 0.20 = 20 % 

On the other hand thermal efficiency is defined as the ratio of work done to the amount of heat absorbed from hot reservoir: 
η = W/Q_h 

So the heat required from hot reservoir is: 
Q_h = W/η = 1000J / 0.20 = 5000J