An object is placed 12 cm from a convex mirror with a focal length of 5.0 cm.

Independent variable is something that is changed in a experiment. 

Is this a question of an answer

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.

The sun's gravitational pull is weaker on the planets that are closer to it, i think.