Efficiency of a Carnot cycle is 75%. If temperature of the hot reservoir is 727ºC, calculate the temperature of the cold reservoir.

The given data in this question is, 

Efficiency η = 75% = 0.75

Hot reservoir T₁ = 727ºC = 727+273= 1000K

To find, Cold reservoir temperature T₂ = ?

Solution - 

For a carnot engine the efficiency can be given as

T₂ = (1-η ) x T₁ 

T₂ = ( 1- 0.75) x 1000

T₂  = 0.25 x 1000 = 250K

T₂ = 250 - 273 = -23ºC