The average speed of the truck is calculated by dividing the total distance traveled in a span of the total time given. From the given above, the total distance traveled is
total distance = 203 mi + 243 mi = 446 mi
and the total time it took for the whole trip is
total time = 3.5 h + 4.5 h = 8 h
The average speed is therefore,
average speed = 446 mi/8 h = 55.75 mi/h
Thus, the answer is letter A.