9.A small object placed in front of a concave spherical mirror gives an image that is real and four times the size of the object. When the object is moved 10 cm towards the mirror, a virtual image of the same magnification is formed. Find the focal length of the mirror.

If the heat engine is a perfect carnot cycle engine, thenefficiency = (t2-t1)/t2   where t1=temperature of cold reservoir ( ° k) = 135            t2=temperature of hot reservoir ( ° k)   = 325thereforeefficiency = (t2-t1)/t2 = (325-135)/325 = 0.5846 = 58%  note: this is the theoretical maximum efficiency of a perfect heat engine.   real-life engines do not achieve this value.