Acar travels 90 meters north in 15 seconds. then turns around and travels at 40 meters south in 5.0 seconds. what is the average velocity during this 20 second interval

Average velocity  =  (displacement) / (total time)

Displacement = distance and direction between
                             the starting point and the end point

The car travels 90 meters north, then 40 meters south. 
So it ends up 50 meters north of where it began. 

Displacement = 50 meters north
Total time  =  (15 + 5) = 20 seconds

Average velocity = (50 meters north) / (20 seconds)

                             =  2.5 m/s north

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.