Efficiency of Combined Carnot Engines

A Carnot engine (E) is working between two temperatures 473K and 273K. In a new system two engines – engine E_­1 works between 473K to 373K and engine E₂ works between 373K to 273K. If η₁₂, η₁ and η₂ are the efficiencies of the engines E, E₁ and E₂, respectively, then